"You" is actually already the second person plural in English. It was also the polite form for the singular, and eventually displaced it in most English speech. The singular in English for hundreds of years was "thou" as in "How great thou art." The direct object was "thee": "I do wish thou wert a dog, that I might love thee something...." The possessive was "thine", as in "drink to me only with thine eyes." There are areas of Britain where the second person singular is still used, and it was commonly used by many American Quakers and Anabaptists (Amish, etc.).
You are boys shouldn't be said like that. You are boys. It should be something like You guys are boys. You = 1 so they're talking to one person saying they are multiple boys. You guys = more than one person is being talked to, which means they are saying all of them are boys.