I think it depends on what kind of school you go to.
I went to a College Preparatory school, and so I wasn't allowed to have a job (thanks to my parents). I babysat on weekdays for about 2-4 hours 3-4 times a week, but that was it. If I had taken a job I would have been incredibly stressed and tired and busy. I wouldn't recommend it.
If your school is more rigorous, I would understand picking up a job. It really depends on homework load, how much downtime you need, and whether or not you think you can handle it.
Sometimes you just have to try it out to learn that it's too much. That's what happened to me in college. I go to a private art school now and my hours depend completely upon my class schedule. Last semester I was able to work 33 hours a week. This semester more than 10 is a strain on my health.
It's all about personal preference. Do you need a day off? Can you run on full all of the time? Or do you need to have downtime, or have too much homework and extracurriculars?
School is the most important thing (as my parents say), so your homework and schoolwork should be paramount.
But again, it changes in every situation.