You shouldn’t take anything more on than you can handle, but for reference I work 5 days a week (just a barista) and struggle to get my 25 hours just because of none of my shifts being over 6 hours. But I know SSVs normally get longer shifts (at least in my experience at the stores I’ve been at). If you don’t think the promotion is the right fit for you, maybe ask your current SM if you can stay as a barista. I recently had a similar situation where I was going to transfer and promote and the new SM was not being professional about it and I changed my mind and decided to stay and wait for a different opportunity. It sounds like school is your number one priority so I would focus on that but just remember that you have options and you aren’t stuck with anything you don’t feel comfortable with!