just say that you need a break from working...like need to rest and have time for leisure and other stuff. or like..how about you might be going somewhere in the summer? if the reason you don't want to work is because they make you overwork with little pay, then when you tell him you wanna quit just say something like you demand or expect salary xxxx and work only xxx days a week? then employer will be like you're kidding and maybe don't want to keep you at all XD


and believe me, staying longer for "better offers" won't do you any good. Besides, you're not quitting cuz of the pay right but because of school work? if you hate it so much now, you will still hate it even if you stay longer with higher pay