Tā / zhōng / xué / bì / yè / hòu / jué / dìng / rù / tuán / ， / jī / jí / cān / jiā / xué / xiào / de / zhì / yuàn / huó / dòng / 。

他中学毕业后决定入团，积极参加学校的志愿活动。

After graduating from middle school, he decided to join the Communist Youth League and actively take part in school volunteer activities.