This whole section seems poorly thought out. I'm just trying to learn Russian, so correctly translating the Russian words should be fine. There is no obvious greater context that this sentence is sitting in so it seems unfair to just expect someone who didn't grow up in the Russian school system to know this.
An F?? двойка is 2. On the grading system where A = 5 etc it should be D. Anyway this system is completely unfamiliar to me and I think DuoLingo should also consider grading systems vary enormously between Singapore, South Africa, New Zealand, Australia and the UK - all of whom will learn Russian through English.
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!