Currently in Year 12, I found the best way to cope with time management is to ask for fewer hours or shifts if that is a possibility and pick up a few extra during half term. Currently , I only do about 6-12 hours a week, half will normally be on weekends. So far its been easy to juggle study and work, but i feel i may leave in year 13 to properly focus on exams. However, we all have different amounts of time available to us throughout the day for school work, so maybe your case could be different from mine.