A quote from the novel that shows Ender is intelligent in Chapter 1.

Miss Pumphrey talked about multiplication. Ender doodled on his desk, drawing contour maps of mountainous islands and then telling his desk to display them in three dimensions from every angle. The teacher would know, of course, that he wasn't paying attention, but she wouldn't bother him. He always knew the answer, even when she thought he wasn't paying attention. chapter 1