This can be made into a very complicated issue, but I think the basic difference for most purposes is this:

"at school" is usually a matter of location. "Where are the children? They are at school." "Did you go home for the holidays? No, I'm still at school."

"in school" is more about activity or status. "What is your son doing? He's in school." "What do you do for a living? I am still in school."

I hope that helps!

This may help:

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http://www.oxfordlearnersdictionaries.com/definition/english/school_1

Read sections/defintions 1 and 2

In section 1... CLICK on "Grammar point"

In section 2... CLICK on "British/American"

The other sections do not refer to/talk about/explain the use of "in or at".

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Remember:

I am 'in school' (NO 'the') means 'I am attending school / going to school / studying at a school.

I am 'in the school' means 'I am in (inside) the building that belongs to 'the school' (WITH 'the')

At school = Where is somebody?

In school = What do somebody do?

Yes,Adam.It helps me a lot, thank you^^

Thank you very much, Richard！The explanation in your link is very clearly！