This song is about Avril and some guy from her high school.

Even though the title says, "Sk8er BOI" I think AVRIL is the guy - all punk and not exactly in the popular crowd  and whoever the "girl" is in the song is actually the guy she may have liked in the past.

The popular guys friends said she wasnt hot and told him not to like Avril, so he didnt admit to liking her because she wasnt hot or popular.

Years later, shes a rock star and the guy and his friends are buying tickets. 

She was a girl
Can I make it any more obvious