Complex sentence: By her senior year, Mary had allowed her grades to slip, and her basketball coach imposed a lockout and her and her teammates whose grades had also slipped; after a secret meeting, the team decided to vent by vandalizing the school gym.
Split into simpler sentences: By her senior year, she (along with other members of the basketball team) allowed her grades to slip, which caused her basketball coach to impose a lockout on her and her teammates. After a secret meeting, the team decided to enact their frustrations by vandalizing the school gym.