"This is not the third grade."
Translation:Це не третій клас.
Quoting the Wiki of the Ukrainian Duolingo developers, even though "є" is technically correct, they want to avoid accepting it as a correct answer because it's misleading to foreigners who are used to using "is" in English and directly translate word by word to Ukrainian. The claim is, saying "є" all the time is unnatural (I agree), therefore it shouldn't be accepted (I disagree) so that the learners learn :)
I don't agree with it, I think it should be accepted (because it's correct! how can you not accept a correct solution!!), just with a note "reminder that saying є all the time makes you sound weird". Or even without any note like that - a foreigner can learn by themselves that it's unnatural! Also, it's only unnatural depending on the region.
So, I disagree with the whole є-ban, but I'm in the minority...