P.M.
I think what makes this sort of totally normal behavior so upsetting to adults is that we have forgotten that childhood innocence and curiosity. They don't know about sex and therefore their play is not sexualized at that age. We don't get why butts are that funny to kids, but they are! Bathroom humor runs amok at this age!
Unfortunately though, Hazel is right. The foolish, idiotic Zero-Tolerance policies at schools may be a big issue, which force us to deal with natural child behavior in a very unnatural way. I agree that you may want to focus your energy on explaining why personal boundaries are so important and why our society does not approve of such behavior.
She is only five and I highly doubt she has the intent to break rules when she does these sorts of things. Maybe she just really enjoys making the other kids laugh (and nothing gets a group of 5 year olds giggling more than butts and boy/girl parts). Maybe grab up a joke book for her age at the library so she has some new humor material. I know this is so hard, and difficult not to take on as your own shortfall, but it's not. Kids are kids and I am sure your daughter will quickly grow past this phase. Good luck momma!