He trains ender to be strong and strategic no matter when the buggers could come at any time, and anywhere so ender and the rest of his fleet have to be ready. The sooner the better.

Colonel graff is in command of watching kids throughout battle school. He watches how ender thinks and his actions. Even though ender gets into many fights he still is recommended for commander.

Colonel graff:

He thinks of the children as soldiers and not as actual children. He pushed them and makes them keep going no matter the cost even if it destroys them.