There aren't going to be many other jobs available for the summer only and if you can find one it is going to take several weeks. So if you need money to pay for college, you should stay at least until you find another position.
How do you quit:
- you can just not show up
- you can call and tell them you aren't showing up
- you can meet with your manager and tell them you are resigning expect that you wouldn't be granted two weeks notice.
You have been there a week..
- you work in a restaurant or retail = shocker that most of the employees change from one year to the next. 100% turnover is typical for these environments. That doesn't mean all employees leave every year. It means some roles will turnover several times during the year. (ie. Waitstaff 1 - stays the full year, but waitstaff 2 - has one employee for a few months and leaves, then another employee stays a few months, and another employee ends up staying for a year)
- you started working again a week ago, shocker that many summer new hires who started at the same time you did are already leaving. They applied for multiple places and got an offer after starting at your place to work somewhere else.
- Sure, they asked you to pick up a couple of these shifts because you are already mostly trained - SURPRISE (not).
- Why do you only want to work 30 or less hours during the week if you need to pay for college?