When talking about groups of masculine persons the adjective endings are different. For plurals this only happens in nominative & vocative (-cy) and accusative (-kich). So the adjective form for niski is niscy and, for another example, wysoki is wysocy.
Shouldn't "These short men work in a school" be accepted as well? I tried "These short men work in the school" in the second attempt and that one was accepted...
I think that the correct answer should be 'work in a / the school' - that's unless the men are actually pupils at that school: I'm thinking back to a prevous exersise 'the girl is standing in front of school', which was deemed incorrect by Duolingo in order to avoid confusion among learners. The word School is a a bit of an oddity in English.