Salutations!
Okay, I want you to listen carefully: Nobody and I mean NOBODY does good on their very first comic. Ever. At ANY time. Does your comic have flaws? Yes. Do ALL comics have them? Absolutely. I'd take your beta readers negative for the sake of being negative comments and junk them altogether. They weren't interested in helping you, they were only interested in tearing up your work and being utterly disrespectful of the love and time you've so far put into the work. So I'll give you honest feedback.
First off, yes, your story could use some work in the narrative department. To start, the first trouble I have is with the changing fonts between speech bubbles. Long story short, you want to use the same font for every character barring intense and unusual circumstance. The uniformity puts your readers into a reading flow and it's subconsciously soothing to a reader (and helps keep them on track). You also want to keep your font size uniform on each page (not necessarily with preceding pages, but just on the pages they're on) Having differing fonts between characters is jarring and bumps me out of my immersion. I would also recommend using shorter dialogue if you're going to have more than one character speaking in a single panel, as you run into the issue of the speech bubbles covering your art.
So that's speech bubbles and text. I'll move on to art: Firstly, it's very clear to me that you love what you're doing. You're making a comic in full color and I know how hard and time consuming that is, not only that, but you're drawing backgrounds detail by detail and trying to make use of foreground and background. I will say that some of your images are blurry when characters look like they're meant to be in the foreground, so for future, I'd recommend making certain that your images are crisp so you don't lose detail.
I will say that I like the way you've differentiated your characters. I can tell who everyone is right from the off and we've only a handful of important leads to start with, and in different locations so they don't all get mixed up. This is a good narrative tool and it helps your flow, so you've actually done a good thing here. I also appreciate that you experiment with different backgrounds and textures. I think sometimes the colors drown each other out, so I'd refer you to this Complementary Colors4 cheat sheet. When in doubt, check it out. I will also give you this2, as you do have fluctuating anatomy from panel to panel. This is a thing that happens when you first make a comic and it's totally okay! You're going to grow, you're going to evolve and get used to drawing your characters. You can refer to the figure drawing stuff I've sent or find some online whenever it suits your fancy!
Now, in terms of story. I will say that I was confused with your first panel, as "Woot" was not a sound I associate with a siren, nor is the color white, so I had to look at the panel a couple of times to get what was happening. I know red is the go-to for emergency lights, but it doesn't have to be. However, using a color that most people associate with danger is a way for your audience to immediately grasp the situation. I do appreciate that you started your narrative in the middle of the action.
One of the troubles you run into is exposition. Now, there is nothing wrong with having exposition but, I'll use an example so you know what I mean:
In the bathroom scene Kristianna is having a conversation with Amy. We go from discussing a bad day to Mike's disappearance. It happens so fast that Mike being missing doesn't register with me. It's just infodumping. Now, you're not a bad artist in the least, and you're good at getting expressions out of composition. You use black and white for intense scenes and I think that, Mike's disappearance, if it's going to be important later, is due a little more of an artistic representation. Maybe show Amy's day in contrast to Kristianna's.
You might try starting off the day with Amy learning about her brother (so we can see how it affects her and how she feels, thus making your audience sympathetic to her plight), moving on to Kristianna in the principals office and then having the two meet in the bathroom to compare notes about their respective days. Then we'd have more of a concept of the pain both of the girls feel.
Now, everything said above, I'll tell you something more important than all of it: Don't stop. If you've got a story to tell, tell it, and even if you don't finish this one, never stop making stories, comics, or art. I think you hit a rash of bad luck with your beta readers if they were so rude to you and unhelpful. The people here that I've met on Tapas are good people who want to help fellow artists for the sake of helping and for no other reason. I hope you have a better experience here than what you had before. Never forget that, you improve forever. You'll never be in a place where your art is at it's zenith and that's the best part, you'll ALWAYS be growing and changing. Sometimes you will get discouraging words, and sometimes you'll get feedback that hurts (and is amicable) so, you'll just have to take both in stride but, don't give up what you love because someone tried to deride you instead of help you.
All of us start somewhere and none of us are perfect. I wish you luck creating your story and I know you can make a good one.
