What does it mean if a guy ALWAYS waves hi even in the middle of class more than once and screams hi to you in the middle of the hallway?

It means that he would be a handful if you ever started dating him. He is going out of his way to make sure that you notice him. In a way, it is sweet, but he is taking it too far. Do yourself a favor and don't go out with him. He doesn't understand the idea of moderation. Imagine how much he'd bug you if you were dating.