。
She takes part in many school activities.
Literal
She [topic-は] many [genitive-の] school [genitive-の] activities [in-に] participates.
参加する ('participate, take part in') takes its target with に, marking the activity or group joined. Same に for joining: 集まりに参加する ('join the gathering'), クラブに入る ('join the club'). The frame 学校の活動 covers everything from after-school clubs to volunteer work and special school events. 部活 (school clubs) would be the natural concrete example.