Summer classes in particular can be very difficult to manage with work, especially if it is a 6-8 week session, and especially with so many classes. It is very stressful to manage both and can be difficult to maintain grades. It is good if school is your priority to put this first and end work if you can afford to do it. For me it was worth it, as I too needed the grades to get into the program I wanted and to keep scholarships as well. My grades improved when I stopped because I had more time to devote to my studies, I could focus better, and I was not so tired. You may find once things settle down, you can pick work up again to integrate back in if you want/need to.

It may be an awkward conversation with your boss but giving 2 weeks notice is helpful to them as you are not leaving them immediately shorthanded. This will help to stay on better terms to possibly to go back in the future. Good thinking.

Good luck with your classes!