Report. I question accepting "a two in math" but "an two in math" is definitely wrong.
Because "F" is pronounced in English as "eff", so it starts with a vowel sound.
From the Tips & Notes "«Едини́ца»(1) is virtually never assigned (maybe only in the case of particularly mighty failure, combining poor performance with misdemeanor)."
A "U" is now more standard in Britain than an "F" for indicating failure - U for ungraded, and can be the result at GCSE (16 year olds exam) and at A level (18 year olds exam). However although duo doesn't intentionally accept U it credits it as a misspelling of two, so can still be used!
I got dinged for "F in math" because I didn't write "an F in math." But this is not a sentence, so I see no need to insist on an article.