It will hurt. No matter what else you have done with a guy, it hurts. It's going to hurt the first few times, too.
Grades don't matter. Saying you 'just mess around with guys' isn't very classy. You don't just go around fucking guys. That's how you get STDs. It's not like making out, where you just get over it. The guy has seen and felt the most intimate parts of you and will tell all his friends about it. Don't do it unless you trust the guy to never tell anyone. Even then, he's going to tell.
Keep the fun hand to genital, not genital to genital.