I'm sure it depends on where you live, but a decent part time job for someone in school is a cleaning position. You vacuum, pick up the trash in offices/cubicles and take it out. You might have to clean off some tables, sweep & mop a cafeteria floor or a main entrance as well. It's easy work for 3 - 5 hours at night. Weekends off in most places. I did this my senior year in high school.
The job usually pays a dollar or two more then minimum wage, plus it's done in the evening/night when the places are 95% empty.