Go to school and go to the school

Go to school

It is used when you are explaining that you are a student.
I go to school every day. ( The person is a student)

Go to the school    
It is used when a person is not a student. The person is talking about the building.
I am going to the school next week. ( the person is not a student and perhaps is thinking about visiting someone or picking up a son/ daughter.