This scene is added to the movie. It was not part of the original script, but was added to soften its otherwise grim and defeatist ending.
Although most of the high school insurgents are killed, a voice-over appears at the end of the movie by Erica (Lea Thompson), (one of the two survivors) showing a World War III memorial, and the American flag flying implies the United States had won the war.
Erica: [closing narration] I never saw the Eckert Brothers again. In time, this war - like every other war - ended. But I never forgot. And I come to this place often, when no one else does.
[we see "Partisan Rock," with its memorial plaque, which she reads for us]
Erica: "... In the early days of World War 3, guerillas - mostly children - placed the names of their lost upon this rock. They fought here alone and gave up their lives, so that this nation should not perish from the earth."
I find this scene very touching.