She is in John and Cameron's chemistry class. According to Morris, a classmate of John Connor, something happened to her the year before John started at the school and she sometimes acts in strange, unexplained ways.
Behind the Scenes
Cheri was originally scripted to play a larger role in the first season, but due to the Writer's Strike and the subsequent rewriting of Season 2, her subplot was left on the cutting room floor. Details remain sketchy, but it is known that she would be the victim of blackmail by the same individual who was behind the Jordon Cowan allegations. John, through his getting to know her, is unwittenly drawn into the situation and his own identity is compromised in the process, placing him in the same position as Cheri. It is unknown how this issue would've been resolved or what dark secret Cheri possessed. 
A deleted scene from "Vick's Chip" shows John investigating her locker, finding the name "Wichita" spray painted on the interior.