Here we go, Ive been looking for a forum to answer this since I was in high school and fully realized the answer. Im also assuming you're just talking about the public school system and not just education in general. (prepare for rant)
The public school system serves a few different purposes in America. It is a system of education in terms of teaching kids facts but it also teaches them how to interact socially with others in a productive and peacefull manner. However my view, and this may just be the oppinion of one of the kids who got sent to detention too many times, is that the public school system is designed to weed out the people that do what they're told from the those who dont. The people who sit down, shut up, and do what they're told are given good grades which is pretty much a ticket to college and in most cases a happy productive life as a cog in the huge corporate machine that makes up our society. The others, the ones that didnt make the grade for whatever reason, have to work a lot harder to get to that cog spot and most never do.
Now for those of you that have been out of school for a long time or are not familiar with the public school system in America you may not know how grades are determind. Well I assure you it is not by judging whether you know the information you are taught. You can pass every test given to you in high school even the standerdized ones but if you dont turn in homework or assignments you will fail the class. School is supposed to be a system of teaching information but thats not really what it comes down to at all. Really its all just about judging whether a person will do what they are expected too in society. Those that prove they can are given the ok to continue on with higher education and upon completing this level are given a higher or more important role in society.
Those that do not do their work for whatever reason be it a bad homelife, personality or social disorder, or they simply didnt want to do their homework are branded with F's for "failing" to fullfill their role as a student no matter how smart they may be or how much information they retained from the class. Some may undergo a change of ways and seek out a community college for access to higher learning but most will wind up in crap jobs making scratch for the rest of their days.
Anyways I would say thats more of an answer to why are we forced to go to school. The short answer to the question why do we go to school is because we have to by law.