。
She takes part in a lot of school activities.
Literal
She [topic-は] a-lot [genitive-の] school [genitive-の] activity [in-に] [participates-参加している].
参加する ('participate, take part') uses に for the activity being joined — 会議に参加する ('attend a meeting'), クラブに参加する ('join a club'). The ~ている here marks habitual ongoing participation rather than an instantaneous event: 'she's an active participant in many.' Japanese school life centers on 部活 (bukatsu, club activities) — sports clubs, music ensembles, calligraphy clubs, English-conversation clubs — as well as 委員会 (committees) and 学校行事 (school events). 'School activities' is therefore a richer, more central concept in Japanese student life than the English phrase often suggests.