It is actually legal to keep students after the bell. There are no direct laws against keeping people in class after the bell rings. Also, your school's policy or regulations can specifically say different rules about the bell. However, teachers must be careful not to abuse their power to keep students after the bel
Except that there's no due process -- you're guilty by being born, and you don't get paid for your work (however few pennies per hour) and you're unlikely to learn a specific trade.