In my country, Brazil, is very common teenagers work during the day and study at night. This young people generally need the money, because their families are very poor and can't afford all expenses. In this case, the student need work, otherwise they don't have possibility of study. Thus, is a good idea work, because working the student will be able to study, and in the future they will have a better job. Other reason is that they get experience earlier. The teenagers learn the things in practice before start the University. This may help them get better jobs in the future.
I have some friends that during the high school have worked. As they start the course in the Universities, they already had a good job. Finally, in my view teenager have a job while they are students yet become them more responsible. I would like explain why. When the young work and study in the same time, they learn to manager their schedule to be able do all the things. As a result, they pass less time playing video game or in the internet. To sum up, although some people think that the teenager students should just study, I believe that is a great idea work while they are in the school.