Alternate Character Interpretation: As cruel as Shelby was, she was tricked into thinking Sam was trying to hurt her. It's less an interpretation of how bitchy she is and more of how justified she might have been in her Kick the Dog moment.
Austin cheated on Shelby with an innocent girl, and rejected Sam for being, well, a waitress. His only redeeming qualities are that he is rich and pretty.
Austin's father was at the pep-rally, and is one of the few people who looks sorry for Sam. This might explain why he got over Austin throwing the game for Sam's sake so quickly.
Fiona may have killed Sam's father. It is at least safe to say he would possibly still be alive if it was not for her.
Double Standard: Sam rejecting/being embarassed by Terry because he is a "loser" during the dance is played for laughs. Austin rejecting Sam for being a "loser" is played out as cruel and wrong, though it is not like Shelby publicly mocking Sam made it any harder to be cruel.
To be fair, Shelby mistreating Carter is also treated as cruel. It is less this than Sam being a Designated Hero. In fact, Sam was being worse than Shelby in that instant. Shelby barely knew Carter; Terry was already friends with Sam, and she knew that he liked her for who she was.
Fan-Preferred Couple: Some fans felt that Sam was better off with sweet, faithful Carter (or even Terry, who already liked Sam for who she was) than Austin.
Harsher in Hindsight: Throughout the movie, Sam is constantly worried that Austin won't like her for who she is. Turns out she was completely right! He wants nothing to do with her when he finds out she is a mere "Diner Girl" and makes no effort to defend the poor girl he just put through hell. Hell, the entire school turns on her for being a waitress.
To rub salt in the wound, there is also a scene shortly afterwards with people laughing at Sam in the hallway.
Hollywood Homely: It's Hilary Duff in a baseball cap! It's hard to believe that Hilary Duff would be considered a "loser" in this at all.
Not really. The only person who ever says she's unattractive is Fiona, who is implied to be jealous of her. That aside, its remarked on about how pretty Sam is by (at least) Carter and Terry, when she's dressed normally. She wears a baseball cap because shes a tomboy who likes baseball.