参加课后活动用英语可以说作"participate in after-school activities".

这是一个固定的短语，来源于学校教育领域中的常用词汇。

以下是一些使用此短语的英文例句和对应的中文翻译：

I plan to participate in after-school activities to improve my leadership skills.

我打算参加课后活动，以提高我的领导力技能。

Participating in after-school activities can help students develop new interests and skills.

参加课后活动可以帮助学生发展新的兴趣和技能。

He always participates in after-school activities, such as playing the piano and joining a debate club.

他总是参加课后活动，例如弹钢琴和加入辩论俱乐部。