I go to school - (in general) I go to a private school I go to the School of Fine Arts.

I go to school - a statement meaning you are a student or a response to a question like "what do you do for work or during the day?" I go to a school - a response to a question of "where are you learning a special topic like car repair?", so "a school" versus "someone is teaching me" versus reading by yourself etc I go to the school - a response to clarify between two or more options. For example, maybe it's possible to work for a company named "SpecialtyStuff" or be a student at a school that also has the same name as the company. Going to the school clarifies which one you spend most of your time at