I kept my job this whole year. I think it defiantly got in the way through, and I know I could have gone a bit better if I didn't have a job. But my hours I work are pretty retarded. Like i would probably work 2-3 shifts a week, but they would always be from straight after school till late at 10:30-11 at night. So it would cut out my whole afternoon and night for studying/assignments, and I would be so tired the next day getting up and going to school after working late the night before, that I would be too tired to do anything that afternoon. Made things a bit stressful, but I don't think i would have been able to survive this year without a job (petrol is so expensive!!), and I have to pay for all my own things and car etc.
I reckon if you do decide to keep your job, make sure your employer knows that school comes first and they give you limited hours. I got burnt for taking off this month for exams. Boss said when he hired me, he was under the assumption that I would be available to work all year round. So maybe make it clear with your boss how much time you will need to take off etc.