Earlier on, Petra tells that Ender that although the teachers gave an explanation for the artificial gravity system, it contains holes and contradictions. From the deceiving and untrustworthy behavior of the teachers, Petra prompted: "Our teachers are our enemies." Later, when Ender first encounters Mazer Rackham, it folds back onto itself, this time as, "Our enemies are our teachers!"
Mazer: I surprised you once, Ender Wiggin. Why didn't you destroy me immediately afterward? Just because I looked peaceful? Stupid. You have learned nothing. You have never had a teacher.
Ender (angry) : I have had too many teachers, how was I supposed to know you'd turn out to be a-
Mazer (whispering) : An enemy, Ender Wiggin. I am your enemy, the first one you've ever had who was smarter than you. There is no teacher but the enemy.