Exactly...it's your first job and so of course you're going to be a bit mortified by your managers (it's a normal thing, don't worry). But you have to get over that. In fact, most managers would prefer that you feel comfortable approaching them with your problems, suggestions, comments, etc. The first job I ever held for an extended amount of time (around four years) was at a book store, and at first I was a bit intimidated by the manager there as well (didn't help that during the week she hired me she had been suffering from the flu and been getting slammed at the store, and so wasn't in the best of moods). However, after working with her for so many years we actually became pretty good friends.
Just remember that yes, managers actually are people too, especially in retail settings. Get yourself out of the management fear rut now or you'll be in real trouble when the corporate world sets in. Your reasons and opinions may not always please your managers, and you're going to have to accept that. However, they'll have much, much more respect for you as a worker and a person if you're able to say, "this is how it is, this is why it is, and this is how it affects you," even if they don't necessarily agree with it, than if you were to find ways to shirk the statement altogether.
So your best bet is just to go up to them and say, "I've really enjoyed working here, but school is starting up for next semester and so I need to give my two week notice." If you think you might want to go back to work there later, then just add in, "I might be able to come back at a later date once I get used to the workload and all, but I don't want to over-stress myself until I know how things are going to be." Nearly every manager has had an employee say that to them before, or has said it to their boss earlier in their life, and so they'll understand. Remember...it's your life and your schooling, not theirs.