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?
所以两者没区别