"You all are boys."
Translation:Voi siete ragazzi.
I believe Voi is plural in itself. Tu is also the same word, but by using it you are referring to an individual. Since English dose not have a plural for the word you, you either assume plurality from the context or situation or a noun may indicate it. I believe duolingo has translated it as you all, to better represent the fact that the object of the sentence is a group of boys.
In this case "You all are boys" would be "Tutti voi siete ragazzi" or "Voi tutti siete ragazzi".
I think that "all" is not translated because English has two meanings for the word "you", both singular and plural, and "You all" could be used to state that is plural, but I am no native English, so I could be wrong. ;)