hi ! im a new employee at a starbucks in canada (started in early september). this is also my first job (aside from like freelance music teaching ive done). when i gave them my availability sheet i said the max amount of hours i can work is 20 hours/week. recently ive been getting scheduled for closer to 25 hours. i know that doesnt sound like that much more, but it basically means i work 5 days a week (unavailable 2 days a week bc class) instead of 3-4 days a week. im a uni student and especially with finals its getting to be unmanageable (less time to work on assignments/study). i also have necessary volunteer things to do (min. of 7 hours a week, for getting into grad school lol), and overall i just have issues with energy and sleeping and the extra hours are kinda messing me up cuz when i get home im too tired to do schoolwork lol. i have a bunch of questions:

1. when i first started i had issues with the manager scheduling me for days that were out of my availability (and had been since interview/hire), and i just spoke to her about it and it was resolved. i was wondering it this is something i can do about the amount of hours ? i dont know if this is something thats commonplace to talk to managers about or if she'd think i was being whiny/difficult bc ive never worked in this kind of environment before lol
2. i put my availability in the sbux partners app we use for schedules, but i couldnt find anything for amount of hours on there ? am i just stupid or is there not an option for that lol. this would be great bc then i can fix it without having to talk to the manager lmfao
3. 2 coworkers recently quit after having 2 separate conflicts with the manager (and her friends/fav employees). i dont have all the details but they were great, it was definitely not on them. they were literally mentioned by name by customers as great employees and were always nice to other employees and customers. that, in addition to the busyness of holidays, makes me think that the extra hours might just be because they haven't been able to hire someone new yet ? so should i wait until after the holidays to bring up the hours issue or do it asap ? on one hand i dont want to make it harder for everyone during the holidays and i feel like the manager might get mad at me, on the other hand the next 2-3 weeks (finals) will be hellish for me if i dont get less hours due to the obscene amount of work i have to do for essays/assignments/exams lol.

sorry for the long post !! i dont really use reddit so sorry if i did something wrong lol, just lemme know so i can fix it.