study in/at

what is BrE?

My apologies. I've amended

1. They study at ABC School.
2. They study in ABC School.

#1 is BrE, while #2 is AmE. Am I correct?

Many thanks.

Sorry... I still don't know what you mean by BrE and AmE...
maybe a native english speaker could help out here.
Anyway, I don't think you should use "study in"....

Do you mean British English versus American English? BrE and AmE may be your teacher's abbreviations. No one else is likely to know them. And that's not a valid distinction between your two sentences. I'm American. We'd probably say "They study at the Baldwin School" (i.e. you're stating what specific school they attend--a school in Cambridge, MA). But "They study in school" (non-specific school, just generically studying), or "They study English in school". You could also say "They study English at school".

I believe it is American English vs. British English.

Anyway as an American speaker... the phrase "They study at Harvard" is the best of the two choices.

I probably would say "They go to Harvard." instead of either of these two options.