I want to stop working until my exams are over, but I don’t want to lose my job if I do?
I've heard this issue a lot. It can be stressful trying to secure a part-time job, so it's understandable that you don't want to lose it and have to try and find another.
My first advice would be to again, try to talk to your boss about it and explain how you need to focus on your study and how you would love for your job to be there when you come back. Hopefully, if you have a decent manager, this won't be a problem. However, we all know this isn't always the case!
If there's a problem and your boss isn't understanding of the situation, you need to make a decision about which is more important to you; your exam results or your job.
Personally, I recommend choosing your exams/results. The likelihood is, that you'll be able to find another job again somewhere, but you won't be likely to find other exam results. That's not how it works I'm afraid!