Technical answer: That is a tradition that started centuries ago as a punishment for kids who WANTED to be in school. Most kids, if they had the option, was manual labor all day, or go to school. Being suspended was a true punishment (sometimes). Now, the justification is that if the kid is sent home, then the parents must watch them, but this is with the assumption that parents want to, or even would (or even have to). But when you have a system that works for the industrial revolution, but still profits old money, you have a failed system. The answer: follow the money.