at school在学校，在上学。在上课（指学生在学校求学）；in the school在学校，在这所学校（指在学校内任职，比如老师等）。at school也可泛指，而in the school则特指某一所学校。
at school含义及例句
释义：在学校，在上课，在求学
例句：
1.He was disciplined at school for talking back to the teacher.
他因为和教师顶嘴受到了处分。
2.Each child had participated in extracurricular activities at school.
每个孩子都参加了学校的课外活动。
3.Her class teacher made a special arrangement to discuss her progress at school once a month.
她班上的老师作了特别安排，每月讨论一次她在学业上的进展情况。
in the school含义及例句
释义：在学校，在这所学校
例句：
1.The pupils assembled in the school hall.
学生在学校礼堂集合。
2.He was the only boy in the school who was known by his Christian name and not his surname.
他是学校里唯一一个被称呼洗礼名而不是姓氏的男孩儿。
3.She is considered the best in the school.
全校数她功课好。