That's hard, man. I know exactly how you feel. For me, it was always when we had to go around the room and say our first name. If they did not know that you had the problem, then you should be happy about than and not worry about the one time out of an entire semester that you blocked. Be proud that you have a stutter, as it is something that you cannot change. My advice to you is to just roll with the punches and laugh it off with them. Try to talk to your friends, explain to them that you have a stutter. I'm sure they will understand.