My friend works in a supermarket on a till for 6 hours on a Saturday and 6 hours on a Sunday.
It's easy for her because she does all her work in frees, in the evenings during the week and when she gets home from work (as she finishes at 4).
However, I know another girl who works every evening during the week at a cafe and has absolutely no time for school work - she tries to do it all at school and is failing miserably.
It's possible if you choose the right hours.