方为介词in at on的用法！

介词的时间用法：

时间点用 at - at 5 o'clock, at noon

时间段用 in - in 1997, in the afternoon, in the weekend

但若是星期几之前用 on - on Monday, on Friday, on Saturday evening

介词的空间用法：

准确的地点用 at - at the front desk, at my desk, at the entrance

在里面用 in - in his office, in the conference room, in the taxi

在上面用 on - on the bus, on the train, on the 8th floor （on 的概念是在上面可以走来走去的）
你的那个例：可以用in 也可以用at
at school
a) in the school building: 在学校
I can get some work done while the kids are at school.
b) British English 英式英语，求学 attending a school, rather than being at college or university or having a job:
We've got two children at school, and one at university.

in school
a) in the school building: 在学校
Sandra's not in school today.
b) American English 美式英语，求学 attending a school or university rather than having a job:
Are your boys still in school?

①表示时