Switching Bodies

"Gahhhhh!", Cassie heard her own voice cry. Instead of standing by the couch, she was now in the doorway, and in front of her was the brown-haired boy and...herself?!
"What's going on?", Cassie said. She realized something sounded odd with her voice, and looking down she saw that she was in the body of the brown-haired girl!
 "That's what I want to know", the other girl said with Cassie's voice, "Who are you? What did you do?".
"N-nothing!", Cassie wavered, "Nothing that should have caused this, at least! I'm running an errand for a friend, and she gave me a teleportation charm to use. I think I ended up in the wrong place, and I was going to use the charm again. I don't know why this happened, unless..."
Suddenly Cassie remembered what she had been told. Whatever you do, don't teleport around a girl named Ellen. The girl had to be Ellen! In a panic, Cassie ran for the door.
"Wait", the boy called Elliot called, "Unless what?"
Cassie couldn't answer, however, because she...