The idea of John Tucker wearing a thong was not in the original script. Originally, Kate talked him into shaving his head. Betty Thomas thought making him wear a thong was funnier. While shooting, the extras were actually taking pictures of him with their phones.
Director Betty Thomas referred to all the actors by their characters' names, since, she claims, she couldn't remember their real names, and she wanted to think of them as their character. When she forgot their characters' names, she'd simply refer to them as "person".
The very last scene between Kate and John Tucker was not originally in the film. It was shot after principal photography had wrapped on a different location because it didn't feel like there was originally closure between the characters.
The song "Hit me with your best shot" is a soundtrack of the movie. Brittany Snow (Kate) also sang it as an a cappella song in another movie six years after this one. It's named "Pitch Perfect". Brittany performed the song together with a bunch other girls. She was one of the back vocals in this case.