Okay, so Joey is about to start his first day at Sky High, the high school for superheroes. On the first day of class, you have to show everyone your superpower and based on its awesomeness, or lack thereof, you get sorted into Hero stream or Sidekick stream. Joey's the son of the two biggest superheroes and so big things are expected of him. Only, he doesn't have a superpower yet. Angst! He does however, have his best friend, JC. Joey and JC have been friends since the first grade. JC's superpower is harnessing nature and making trees and flowers appear and grow and stuff. JC doesn't like to use his power for to hurt others, and he doesn't believe in the class system either, so he ends up in sidekick voluntarily. Joey has to admit he has no power and it's humiliating and he's a sidekick too, only he can't tell his parents. But JC is there to comfort him. Nick is another friend of theirs who ends up in sidekick, though he thinks his awesome ability to glow in the dark merits hero status.
Joey ends up admitting to his parents that he doesn't have a power, and might never get one. Of course, they are incredibly supportive, as Papa Fatone loves his Joey. As it turns out, the whole class of sidekicks are a great bunch of kids and Joey fits right in. There's Lance, who can turn to goo on command, and AJ who's a gerbil shapeshifter. But they constantly get picked on by the nasty Kevin and [someone else, possibly someone from 98 degrees]. And Joey has an arch nemesis, Chris. Chris is the son of a hero and a villain, who Papa Fatone took down. Chris is all scary and a loner and can throw fire and stuff. And when he and Joey finally come to blows and it looks scary for Joey...he gets his power!! Super strength! He is thrilled and so are the parents and he gets switched to Hero class.
This is where he gets to hang out with his crush, Justin. He's the most popular girl in school, and Joey has a crush on him. Justin tutors Joey in science type things and they're planning on going to homecoming together (or maybe it's called the hero ball? I can't remember). This is devastating for JC, who is totally in love with Joey. JC is also perfect for Joey, but he doesn't notice, of course. JC vows to make Joey jealous by going to the dance with Chris. It turns out Chris is really a good guy, and of course JC can charm him into doing anything.
Justin is totally leading Joey down a popular path of destruction, making Joey leave behind all his sidekick friends, which Joey doesn't want to do. Finally, Joey realizes this and ditches Justin...but is it too late?
Yada, yada, yada, Justin turns out to be evil, Joey must save the world with the help of his sidekick friends and Chris, and most importantly, JC and Joey KISS BECAUSE THEY ARE IN LOVE. The end.
And if you haven't seen this movie, you really should. It's adorable and funny and sweet.