Other mistake: In the scene where Nicole opens her locker and a note from David falls out, the handwriting is exactly the same as the seen where Nicole leaves a note taped to the bedroom door for her father. Although the letters are larger, the same applies to the note on the father's vandalized car, which says he's popped both of their cherries.
Other mistake: In the scene where Gary and Nicole are leaving school and Nicole is telling Gary "please don't tell Margo'' when we first see them they are wearing clothes that were worn in the ending then it switches. Keep eyes on Nicole at first her clothes is the black dress with pink shirt underneath and her hair is down, it switches and she's wearing red pants, purple shirt, and hair in a pony tail.
Answer: I always thought it was a girl giving him a blow job.
Me too! I've seen it more than once and it's not until I'm watching it just now that it shows up as a dog, I could have sworn it was a girl.