Do you have a specific question?
Like Curfew, I'm not sure what your exact question is, but if your penis is 6.4 inches while erect, you fall right into the average range. If you take time to make sure your gf is properly lubricated (wet), and do plenty of foreplay, or use lube like KY Jelly, and go slow, especially if she is inexperienced, it shouldn't hurt.
The erections in class, well, that happens at your age. That will get better. Try to pay attention to what you're doing and it might not happen as much.