What was with the dead schoolgirls in the film Atonement?

I asked this before, got no answers but I am very curious.

There's this scene in Atonement when Robbie (James McAvoy, whatever)
is walking through the forest with his two friends on the way to the
beach and he comes across all these dead schoolgirls, I'm guessing,
lying on the forest floor with clear bullet holes in their foreheads.
Robbie looks really devastated and distressed.

I was wondering, what happened historically? Or is this scene
symbolic?