Focus on school if it's that hard. You can always do your sketches on your free time. Even 30 minutes a day is sufficient. Don't forget you have to think about life after high school too. Plan things out first.
I'm battling university and comics at the same time right now... Yes I'm very stressed and yes I'm barely scraping by on some fronts, but making my comic is also so rewarding for me I can deal with that. You have to consider what works for you and I think the best way to do that is just to try it. Try get started and see how it feels, build some kinda buffer and then consider if you can continue. You can always start over again later!
There are lots of options here that aren't choosing between continuing your education and working on your comic. My personally feeling, though, is that school is more important. It depends what you want to do for a living though. Comics can take 5 or 10 years before they amount to anything. Most of us are going to school and or working jobs while we make our comics, because it's a long ongoing process.
- You can take a gap year and spend that time working on art/comics.
- You could go to school part time (city college, state college, or university), and thus balance school life while making decent time for art. Plus you could probably take an art class!
- You could go to school full time, but try to make an art schedule that allows you appropriate time for personal projects.
- You always have summer, winter, and spring for art!
Focus on school first!
It's easier to spend your youth and energy on classes and coursework now than later.
See, a comic requires planning, most especially if you want a long-form story with arcs and a solid plot. I think during school is the best time to really develop your comic and figure out the story you want to tell. It gives you a break from academics, and you can use whatever knowledge you pick up in your classes and apply them to the story's development. Who knows? You could get inspired by a history lesson or physics principle.
While you are at school though, you can start posting artwork and snippets of your comic development to begin to build an audience, then when you do have the time to devote to an actual comic, you could already have fans.
why not both?
maybe you don`t have the time to work on a full comic, but you still can sketch, practice and generally have fun drawing. You just gotta make yourself the time (after homework hopefully, be responsible 
)
Also, you can use what you learn in school for inspiration for your stories
I'm still in high school and yet I manage to make time to have a consistent schedule for my comic while maintaining good grades. it's not impossible. to be honest, studying isn't really necessary if you just pay attention in class and remember what you've learned.
Looking back I wish I had started working on my comic when I was at school. Now I have to juggle my work and the comic, I learnt that I will never have free time so I have to combine my hobbie with whatever I have to do in life. I don't think school and comics exclude each other. If you can't draw daily, then draw whenever you can but don't abandon. Definitely prioritize school but you can start with your projects.
I’m a highschooler too, but take my advice with a grain of salt because I haven’t updated in 3 weeks.
Before you even start your comic, I’d suggest waiting until summer/winter/spring/Thanksgiving break and create a really big buffer. That way you can update it consistently over the school year without worrying so much about putting out an update every week.
DO HOMEWORK FIRST
Making comics is a hobby (for most) so it should always take second priority to school. If you have an exam the next day, you shouldn’t work on your comic instead of studying. Also, it’s rewarding to finally do what you want after finishing all your school stuff.Manage your time... if you’re an aggressive procrastinator like me, clean yourself up before you add comic-making into your schedule. Learn how to manage your time and spread things out. Fitting in an extra 30 minutes(or so) if drawing every day should be easy after that.
Good luck 
As a wise terrible website by the name of tumblr once said, "It does matter how slow you go, as long as you do not stop."
Or I guess Confucius said it. Whatever.
Anyway, I don't think art and school should be entirely separate bubbles where one cannot touch the other. It's not like if you decide to dedicate a little more time to drawing, your grades are going to tank.
You will probably be busier during university. If you want to continue your hobby of art, just find time to do it when you can. There's nothing wrong with only drawing one page a week, or even a month. And even if you turn out to dislike that page in a few years, you can scrap it and do it again with the knowledge you gained from doing that page in the past. You can just doodle too- In class free time, at lunch, etc. I managed to draw a lot in middle school/high school by completely abandoning watching TV (I never really enjoyed it anyway) and instead using that 2-3 hours to make art. I say to at least dabble in working on your comic so when you feel like you can dedicate yourself to it, you have some stuff to work with already.
The way I worked when I had school was I would create a buffer during my vacation time for the whole semester and then casually edit during lulls in my schoolwork.
I am a novelist so I'm sure that doesn't translate completely, but you could make some simpler styled comics and make a buffer and then make it more detailed when you have lulls in schoolwork.
What is your purpose? Do you consider making comics as a hobby or as a future profession? How much are you sure in your decision? The answer on the stated question depends on that.
I think it's a very personal choice but I also think it doesn't have to be one or the other.
I started my comic when I had lots of free time in my hands but now I'm in university and it is getting quite difficult to always find time to draw. However for me drawing comics is something I enjoy greatly, so I want to make time for it. I draw when I take a break from schoolwork and drawing actually helps me relax. However sometimes I do need to take a break from comic work to concentrate on uni and that's okay too!
Life is all about finding a balance between things you have to do and want to do. If you really want to draw comics then go for it! Find a balance between school and art that works for you.
Yeah, I mean, I started when I was slightly younger than you and I'm doing monthly updates even in exam seasons, but if your going to do it do 10-15 page monthly or short weekly updates and you'll probably be fine. Actually I'd do Monthly if I were you, because attention shouldn't be you first bet when making a comic and you won't get alot of it but you're given time to actually consider your story and how everything flows into it, then you can improve, while you have the free time to take a week or two off out of the month. So if you do 10-15 pages monthly, not only do you have enough time to plan and improve, you also do about 2x-3x more than the average creator on here. Also, break months are a lifesaver.
What I wanna know is why do you ask? What type of art do you do that you can't just take some paper and a pencil in your rest-times and let out whatever you want to draw? 
This is probably the truest statement here. There will NEVER be an optimal time to work on it. If you're passionate, you have to make the time.
However, this is also good advice.
I work on my comic while working full time. In some ways its like working after I get off of work. But, I don't mind. It's what I want to do.
However, I took a long time to build a buffer, and used my vacation time as periods of getting work on the comic done faster.
This worked quite well for me, and has saved me from missing updates during periods where I can't work on pages at all (like right now).
Just make sure that you prioritize your school work. Though you may love your comic now, when you're finished you'll believe you can do something bigger and better. Your future inspiration, and writing, will come from your education. Not to mention, your education is what will pay your way to your financial future!
To be honest, I would try to find time for both!
I was drawing since I was little and especially during graduation time in high school, drawing helped me relax a little from school stress. I would recommend to do drawing "just for fun" while you're still in school - while sketch-a-days are a good general practice, it's not something for everyone (I don't do well with these either.)
If you want to try yourself on a comic, it can be something you could work on, however, I wouldn't neglect your academics because of this. Maybe just work on it whenever you feel like it but wait a little until you gathered some pages before posting if you're concerned about regular updates.
Art doesn't run away, and I'm sure you'll find enough time to draw even in university. I really just recommend balancing both.
School is important! You can open more paths for your future that way! Doing comics can be one of those paths that you can pick in your 20s! But I'd you focus on comics and let your grades drop、 your options later in life will be limited to comics and whatnot. Better to be safe than sorry!
I agree fully with (and am in the same boat as) @amortelito. I drew quite a bit (and worked on kinda) a comic in highschool, pretty much took a few years off of drawing for fun in college (I studied architecture so I still drew a lot.. just buildings, instead of characters xD) and only started working on my first "serious" comic after I had been working full time for a year.
In short I would highly recommend you "focus" on academics, for the reasons many listed above. Although they're not strictly required for every job, many of the best require them (and they just generally open a lot of options). Buuuuut going back to the first point, do drawing/comics too! Time management is hard and doesn't really get easier. You just gotta exert some discipline and make it fit.
Now then, some general tips:
- Find a schedule that works for you. There's a reason many hobbyist comic artists only upload a page a week or sporadically. We're busy with our other responsibilities too. Don't stress if you can't match someone else's pace. Find your own~
- Do try to get into the habit of drawing every/most days. No matter how much free time you technically have, this is difficult to manage. Especially if you have other hobbies. I know you said a sketch daily thing wasn't working out for you, but the tool i used to help find my balance was inktober 2017. First one I did, and just figuring out how to squeeze drawing into my schedule every day was helpful, for me at least.
- That said, don't be afraid or feel bad about taking breaks! Inktober is the only period I draw legit every day (2018 was intense because it was my first one working and doing my comic at the same time xD 2017 was just work), the rest of the year varies. I do have to draw most days to upkeep my comic schedule, but there are many days where i meet up with friends after work, or on the weekend, or are just lazy and would rather play video games. That's totally cool~ the only time it's not is if drawing is your job xD
- i like this advice for all newbies, but consider starting with a small project or two (or three, or ten) no matter your time situation, hopping into a long multiple chapter story right off the bat is daunting, and comics in general have a big learning curve! Your first pages will definitely be your worst and least efficient, so consider getting those "warm up" pages out of the way in a self contained short story (s)! Write something you can complete in a few months: you'll learn a lot and probably feel good about having a finished project in hand too :>
I think that's about it. Most people manage to do school/work and drawing at the same time (at least at a hobby level), so I believe in your ability to as well! Best of luck!
I believe that you can work on school first and comics second. I would take a certain approach to the dilemma, if I was in your situation. I would work on my school work with the proper amount of time and mental focus. Since you draw when you are inspired, I would only draw when inspired and time permits. I would right this down in a journal. Persistence is the most important thing in this situation. Usually improvement comes with discipline to follow out a plan. In time as long as you continue to review and update your schedule, your plans will change and your art work schedule will evolve to fit what makes you happy.