Ⅰ. at school表示“在学校、在上学”相对于在家里或在校外。如：
① My son is at school now. He is not at home or somewhere else. 我儿子现在在学校，他不在家，也不在别的地方。
② When my brother was at school, he studied very hard. 在学校时，我兄弟学习很用功。
Ⅱ. in school “在求学、在上学”相对于有工作。如：
My daughter still in school She doesn’t work.。 我女儿还在上学，她不在工作。
：①和②用at school 强调所在场所或时间。③中的in school 则强调主语的身份是学生。因此，in school. 和 at school的着重点不一样，通常不互换使用。
Ⅲ. In a / the school “在学校”，不一定指上学。类似的还有：
in hospital “生病住院”
in a / the hospital表“在医院”(工作或探视病人等)
at table “在吃饭”
at a / the table “在桌边”(有可能在聊天或看报)
① Is your friend in school? 你的朋友在上学吗?
① Your friend looked for you in the school just now. 刚才你的朋友在学校里找你。
② Children are often in hospital when they are young. 孩子们小的时候经常生病住院。
③ She is a good doctor in the hospital .她是医院里的一名好大夫。