Considering quitting... (huge rant!)

For reference, I am 16 and working my ass off in school rn. I work maybe once a week because my store cut down hours to shit and instead of listening when I say I can't work, they try to schedule me on days I've already said to my manager "I can't work this day" sometimes a week to weeks in advance. A lot of family stuff/plans of mine (ex. Seeing my dying grandfather) I'm not told about until soon before it happens because my family has poor planning skills (not the stores fault) but it's usually still weeks before it happens, yet bc it's not far enough out I have to directly tell my manager I cant work instead of requesting time off online. My coworkers are sweethearts but I know some of them have a distaste for me because I have to call out a lot.

The pay is also shit for how much work they have us doing! It's crazy how I come into work and I'm told "Okay so I need you to refold all the shirts, do this huge u-boat, etc etc in the next 4 hours." If I was working a 6-8 hour shift I totally understand, but 4 hours?? Along with that, I have explained to my SM multiple times that I have really bad social anxiety, and while I'm great working with customers out on the floor, I quite literally cannot do register because being thrust in front of a ton of customers at once is horribly stressful for me, and I cannot understand the register whatsoever!

My orientation day they shoved me onto the register in the middle of a rush WITHOUT EVEN HAVING ME DO MY PAPERWORK FIRST. What the hell is that?? My information wasn't in the system yet so their decision was to shove the new girl onto the register in the busiest hour with no training, no paperwork done, nothing, and they were surprised when within 10 minutes I had to tell my coworker to take over and I ran off to the bathroom bawling my eyes out. My SM wants me to start on the register by summer, and I really don't know how to tell her I still can't. My "first day" ruined my expectations of the company and of register work and while my social anxiety isn't the stores problem, I can't be shoved in front of a bunch of people like that and that part I just cannot do. It doesn't help that they often choose to schedule me with one or two other people the entirety of my shift and I have to ask my coworker numerous times to head to the front to check out a customer because I can't, and they shove whoever's working with me on 2-3 tasks at once.

Most days I can handle the work, but I feel like this is really not worth $12/hour. The only reason I've stayed on so long is because I don't know any other jobs around in my town that would be accommodating for how bad my social anxiety is.

If anyone has some advice or ideas on what to do that would be extremely helpful!