She and Maggie have similar haircuts. According to Nicktoons, they are the same person, but this is inaccurate. In a scene in "Vegetables", the pink schoolgirl is seen in the background and Maggie is seen in the foreground. Also, in the episode "New Best Friend", the two are sitting next to each other.
The pink schoolgirl has never said anything on the show; she only screamed in "Vegetables". The scream was most likely provided by Amy Winfrey.