If you are a student, then you would "attend school" (without the "to"). If you are a parent going to a meeting at the school, or just going to pick up your kids, you would "go to the school". You can also say "I go to school" (as a student). And to make it even more difficult, if you were a parent, you could attend a meeting at the school. In a way, it does have to do with frequency. As a student, going to school regularly for the classes held there is the key. As a parent, it is just a location where you are actually doing something else, like picking up the kids.