"You are boys."
Translation:Ni är pojkar.
Du is only used for the singular. Since you're referring to a group of boys, you'd use ni instead
I have been told by Swedish teachers that "ni" can also be used to address single persons formally. But nowadays that method is mostly used with elders or possibly people in high authority. I could be wrong. But I have heard it said.
Your post is over a year old and you probably know this by now, but for the benefit of other learners: That's largely a myth, though a common one. The word ni was mostly never in use in that way - Swedish used a titular system instead.
Gosh ni and du frustrate me but thank you to the people who explained the differences