"Going to" often expresses an intention for a future event, so yes, your first statement is correct. "I think I will go to school tomorrow" expresses more doubt. Constructions that being "I think I will..." often indicate that there's a choice (e.g., "I think I'll wear this outfit tomorrow." or "I think I'll have a Coke.") "I was going to go to school tomorrow" infers that the speaker has decided to do something else and is usually followed by a qualifying phrase, "... but I decided to go to the movies instead." Here's a link describing simple future constructions (I will vs. I'm going to) http://www.englishpage.com/verbpage/simplefuture.html

"going to go" is tautology