Translation:Forty-seven teachers are working in the American school.
What is the difference between "47 teachers are working in the American school" and "47 teachers are working in an American school"?
The az here is important, marking that it is a certain school ("the school"), instead of just any one school ("a school").
I said, "...working at the American school," which was rejected (in favor of "in the American school"). Fine, except that there was another sentence in this lesson where I wrote "... works... in the ..." (the pertinent words were "dolgozik" and <someplace>ban/ben. (Sorry, my memory is terrible. It was maybe two translations before this one.) It was accepted, but also gave "works at the..." as an alternative. So sometimes -ban/ben can be translated to "at," depending on the context - I get that. But the two sentences I'm referring to had very similar contexts. Should I report it?
You should report it. "Working at [somewhere]" is an appropriate translation for dolgozik vmiben.
Exactly. You'll see that kind of thing in a dictionary, when it gives an example of how a word is used (especially verbs). "vki" = "valaki," "vmit" = "valamit," and so on.