202: I think the idea was that since all the teachers know that Ellen is really Elliot, if any of them heard she was in the locker room enjoying the show they'd assume the worst. She's not supposed to be in there. Still, Ellen's thinking in terms of sitcom-level "evil twin mischief" rather than body-snatching-and-death level retaliation.
200-204: Yeah, basically Ellen thinks this is a sitcom and "evil" means "getting your enemy detention for a day or two and maybe forcing an awkward conversation with his girfriend". It's incredibly depressing that she wanted this to be her dying legacy.