A couple of thoughts:
1. Working is a way to learn about the world in a low risk way (losing a job as a teenager is often more a lesson than a set back, for example). It’s also a way to earn your own income, which is obviously useful.
2. Not every teenager has the luxury of living in a stable home with one or both parents/guardians. Even then, some teens are better off spending more time away from those people beyond time spent in school.