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.