The point where Gary could have saved himself a lot of trouble
When he gives Karen a ride to school and then runs after her all nervous to ask her out...."sorry I'm busy." "that's too bad."
The end.
She wasn't interested.
I know it's Monday morning QB and that you don't have the wisdom at age 16 to figure that out, but it was clear right there that she appreciated him giving her the ride but just wasn't interested in him. She liked a*(&holes. He should have moved on.
Also, he makes it worse by pouting all the time. If he acted like he could care less, he'd at least have a 7% chance with her. The whining and hangdog face just made him less attractive to her. Also, if he had plowed Rose (she was better looking anyway and was probably a wilder lay), that would have upped her chances with Karen to like 30% because it was her best friend, she'd be envious and wonder what she's missing, plus she'd see that he could care less about her and was happy plowing Rose. Gary went about everything the worst possible way, but we've all done this to some extent with a girl we're infatuated with.