First reason why children go to school is to get knowledge in different sciences. Teachers on schools are there to give children explanations and to teach children different sciences, such as math, language, history, art etc. Children are leasing the lesson and they do home works and test to determinate there knowledge.
Second reason why children should go to school is to get some social skills. In school children spend lot of time with other kids which is in similar ages. Children have contacts between themselves and they learn how to communicate, how to play with other kids and how to behave in society.