old black and white movie

I saw this movie on TV back in the 1970's. It is either a sf or spy type movie, the only scene I remember is an army truck with a a bunch of teens in the back pulls into an army camp. The soldiers start ordering them off the truck and they open fire with a machine gun. One guy is trying to hold on to the canvas on the truck and a soldier forces him off and he gets shot with the rest. Two officers are watching and one I horrified and says " they were just kids". I remember being shocked by this as a child. If anybody has any ideas please post.