In the beginning of the film, their teacher talks about how great the war is and when they finally go to war that is when they begin to act differently about how the war actually is.
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There are a couple of versions of this film. Still, your question seems standard to the story as well. By the end of the war, they no longer see the glamor and adventure of war. They see through the lies of their teacher Kantorek. The see firsthand the devastation that war reaps on both sides. These boys no longer care about patriotism or Germany: they just want to make it out alive.