In swim club, when Chrissa told Tara to leave Tyler alone, she was on the diving board ladder. When Tara said no and turned back to Tyler and when the camera went back to Chrissa, she is off the ladder.
Towards the beginning of the movie, it's established Chrissa's first day of school was on a Friday. At the end of that school day, the teacher announces swim club permission slips were due the next day, which would be a Saturday, a day school is closed.