Translation:After seven months of school, he is finally counting to ten.
"after seven months of school he finally counts up to ten" .. tahle možnost?
I agree with albeck's interpretation with finally at the beinning of the clause. It seems more natural in English. It should at least be accepted. The duolingo answer seems unnatural and stiff although in general usage.
"After seven months IN school, he is finally counting to ten." Think you can use IN as well as OF in this sentence.