There are a few places that we think of as "states" more than places---we are more interested in what we do there than in where they are. For these words, we usually don't use "the".
So I will go to THE post office and THE store.
However, students don't go to THE school--they go to school. Even if they are on a field trip, and they are not going to the physical school building, they are still going to school. Similarly, we go to work, go to church, go to jail, go to prison, go to bed, and go to college. In England, people go to hospital and go to university, but in America we say they go to THE hospital and they go to A university.
This is true whenever we use the words on the list, not just with "go to". If you ask "what time does the school start", you are asking about one specific school, and contrasting it with other specific schools in its city. If you ask "what time does school start," you are asking about the moment when children across the country change from being "at home" to being "at school".