how are peter and valentine unknowingly changing enders training at battle school

It is dramatic irony when Graff calls Demosthenes and Locke, and therefore Valentine and Peter, "short-sighted, suicidal people," and that he implies that pushing Ender "to the edge of human endurance" is too much of a sacrifice in order to save the lives of such people. If Graff knew who the writers actually were, of course, he would know that Ender would want to save Valentine, at least. Valentine is, in fact, the only person who can help Graff, who comes to her more than once in order to convince Ender to keep pushing himself. Valentine is Ender's one real connection with Earth.