How to figure out the meaning of at / in school

I am really surprised and confused that even many native English teachers have different thoughts about usage of 'at school' and 'in school' and and they say it is an American version and British version respectively, but they are mixed as well. Then how can I figure out what it means in conversation or in writing? I think that first of all we have to use context and if we cannot figure out exactly what it means, ask back or just let it go because whether it means 'inside the school building' or participation in activities associated with a school' sometimes it does not matter so much. How do you native English speakers figure out those problems?

A : Where is your son now?

B: He is at school.

It can mean "he is now inside the school building (British version) or "he is taking classes in school now", right?

A: Are the kids in school at the hour of the day?

B: They might finish school earlier for some reason.

When we see kids strolling around on the streets, we can say the first and it can mean "inside the school building" or "they are taking classes now" whether it is an American or British version. Or regardless of the version thing, in the context, we can infer the two different meanings, can't we?

Then do we have to make it clear or we can let it go because there is not much difference or they are not of importance? How do you figure such things out?

Thank you so much as usual and I really hope to hear from you.