The Instant of Now by Irvin E. Cox

The Vininese drove Dirrul to the city in a heavily armed surface car. Two of the infantrymen sat behind them, their rocket guns ready on their knees. It was testimony to the efficiency and organization of Vinin that such a finished reception could be prepared on such short notice. Dirrul's first intimation of the scope of the ceremony came when they stopped at a school to be cheered by the pupils.

Rank upon rank of boys and girls lined up smartly behind the high wire fence. They ranged in ages from tots, barely able to stand, to young people in late adolescence. Except for the round metal disks, which all of them wore, they were completely naked.