Here's the thing I know for a fact about retail/fast food jobs. Have only as much loyalty for them as you need to keep your job for as long as you need it. They have 0 loyalty for you. Those low level jobs are the hardest. They work you like a dog, and then spit you out.

Don't let the other staff walk on you. If you can't take a shift, you can't take a shift. If you can't come in early due to class, then you can't. Class is more important because it will lead to your career. Be a good employee but say no when you need to or you'll burn out and that will effect your school. You don't want to be trapped working retail/fast food forever.