Hey Alex
I had a job in Year 11, and in my personal opinion: it was terrible, in the sense of balancing it with study.
As a result, I quit in Year 12.
I had to have two to three shifts (sometimes four!) a week, and it always tended to be on weekends. Unfortunately this was the only option they had for me. In my case I tended to do homework on weekends, and relax during the week.
It wasn't good for my study, I started staying up until the early hours of the morning to complete homework on weekends. I wasn't able to relax on Saturday/Sunday during the day because I had to rush and finish any school work before my shift.
The strange thing is: In year 12, since I quit, I literally had 12-16 hours EXTRA in my week. Yet I personally don't think it has advantaged me much at all, in that I haven't used it all productively. It did give breathing space, so that instead of going to work, I would watch TV, go out, play games, catch up with friends etc.
It goes both ways and it is up to the individual.
What I could suggest is that you apply for a job and be extremely clear you will ONLY work ___ hours a week (I would say 1-2 shifts is ENOUGH, anymore means you are not focusing on your studies as much as you could!).
Also, take 1-2 weeks off around/during exam time.
I hope it has helped! And I know the pressures/desire to work can be difficult during your most important years of schooling! But it all comes down to balance.