Untrained is the operative word here. I do remember mugging an adult instructor with 2 other buddies (one went on to become a Paratrooper and the other a Rock ape) in the army cadets aged about 14 or 15. But this was in sand dunes at dusk, so there was an element of surprise, which we later got in trouble for because he lost his Enfield bolt rifle, assuming that we had run off with it. We did receive CQB training for the 3 star qualification aged about 15, which was a team activity i.e. a battle formation of 3-4 working together. Organising a response of untrained teenagers would be virtually impossible and dealing with/killing/disarming the assailant would be virtually none existent, especially if the camp had a no knifes rule in force.