Answer: When Aaron tutors Cady
They only kiss three times in the movie. The first time is when Aaron tutors Cady even though she is an excellent math student. It is right after, that Cady tells him that Regina is cheating on him, and he breaks up with her. The second time you would think that they did, in Cady's room the night of her party, they don't. They almost do, but they don't. The third and fourth times are both at the Spring Fling.