The first sentence “I go to school” suggests that you are a student. The noun “school” when used without an article indicates that you are thinking of school as an institution, as a place of study, rather than as a building.
Similarly, when you say, “Mohan was in hospital when the fire broke out” what you are implying is that he was a patient, and not a visitor.
In the sentence, “I go to the school every day”, you are thinking of “school” as a building. Perhaps, you have a child in school and you go there every morning to drop him off. You are not going there as a student.
When you say, “Mohan was in the hospital when the fire broke out” what you mean is that he was in the building, but not necessarily as a patient — perhaps he was just visiting someone.