Novel in which aliens invade Earth, human teenagers look very similar to the alien teenagers

Question

The aliens have been winning the war and the humans realize that their teenagers look similar, so they train teenagers to be warriors to infiltrate their camp. On the way in, they are ambushed and the captain of the squad got separated from his team and knocked out.

The team believes their captain is dead, but they have to continue the mission. By the end of the book, it's found that their captain is alive and has been found by the enemy, but the one that is caring for him knows that he is human and won't tell anyone.

Answer 1

I initially just gave this suggestion in a comment, as I felt that some of the more significant details did not match the question. But from a reply posted by the OP it seems the answer is indeed "Assault" by Brian Falkner, the first book in the Recon Team Angel series.

The story takes place in a future where the Earth has been invaded by an alien race, the Bzadians, and the remaining humans are bunkered down in the Americas. The Recon Team Angel are a Special Forces unit composed of teenagers, recruited from around the world, who crucially are the right size and build to be disguised as aliens and infiltrate their base in Australia (actually at Uluru).

What I did not recognise is the detail given in the question "The team believes their captain is dead, but they have to continue the mission". From what I recall, it is rather the case that the team contains a traitor, a human who has been brainwashed by the aliens, and they have to try and discovery their identity while completing their mission.