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?