This is the 290th most frequent Chinese word.
留
"Leave behind," "stay," or "retain."
Here, '留' indicates staying back or being made to remain in a location, in this case, school.
他被老师留在学校完成作业。
He was made to stay at school by the teacher to complete his homework.
In this context, '留' is used to mean leaving information or a message for someone.
你能帮我把电话号码留给他吗？
Can you help me leave my phone number with him?
In this sentence, '留' means to let grow or keep, as in maintaining long hair.