None. No Sex or Nudity. Some kissing.
We see Eric pantomime sexual activity on a bench in a joking manner.
Eric mentions something about being ready to "get his freak on" and one of the girls tells him to "put your freak away before you hurt somebody." We also see Dean make a sexual gesture toward one of the girls by putting his hands on either side of his clothed crotch and then doing a pelvic thrust. Eric then mentions that he's ready to get his "groove on" with Belinda (but she blows him off).
A photoshopped picture of Jaime circulates around the cafeteria. It is of a woman in a bikini. However in the background on the flyers the other students are holding, the original picture (of a naked woman) can be seen faintly. (The director intentionally had the camera see only the bikini pic clearly and not the more graphic version in order to achieve a friendlier rating.)
When Jamie and Landon are looking at the stars he pulls out a blanket and Jamie asks him if he is going to seduce her. Then he asks her (as a joke) if she is seducable. She answers no. It goes no further from there.
Eric suggests that a female teacher had sex with the student who wrote the play that Jamie and Landon are starring in. He thrusts on a bench implying the two people having sex, impersonates both in the situation.