Both could be correct depending on the context, although I would more naturally use the verbs :got to' or 'arrived at' If you say 'reached', this makes me think that this is end of a difficult journey. If you say just 'school', this is your usual school where you go every day. If you say 'the school', this implies it is a different school or you are not usually a student.

"the school" is always correct if you are trying to reach a specific school. "school" is if it's your regular school that you always go to. (similar to how "home" can omit "the") "the school" will work in both circumstances