Toward the end of the book, Ender is moved out of Battle School for his final "training." Mazer informs Ender that instead of toon leaders Ender will now be working with squadron leaders.
As you worked with toon leaders in Battle School, so now you will work with squadron leaders.
Ender then asks when he can meet them, and Mazer tells him that there will be no such meeting. Ender wants to know why that is so. Ender believes that he needs a face to face meeting with the squadron leaders in order to size them up as a person.
"How can I work with squadron leaders I never see?"
"And why would you need to see them?"
"To know who they are, how they think—"
"You'll learn who they are and how they think from the way they work with the simulator. But even so, I think you won't be concerned. They're listening to you right now. Put on the headset so you can hear them."
Mazer doesn't believe that it will be a problem. He is confident that Ender will be able to learn the tendencies of each leader through the simulator training sessions. Mazer also knows something that Ender doesn't know at this point. All of Ender's squadron leaders are his former friends and classmates from Battle School. As Ender puts on the headset, each squadron leader checks in through radio, and Ender is pleasantly surprised to find out that they are his trusted comrades.
And Petra, and Dink; Crazy Tom, Shen, Hot Soup, Fly Molo, all the best students Ender had fought with or fought against, everyone that Ender had trusted in Battle School. "I didn't know you were here," he said, "I didn't know you were coming."
Each of the squad leaders performs as well as Ender expects them to. He knows their capabilities and tendencies. The main issue concerning Ender and the rest of the children is exhaustion. Ender pushes all of them to the breaking point, and he has to rotate through them to keep them fresh and rested.
Sometimes battles lasted for hours before they finally destroyed the last enemy ship. Ender began rotating his squadron leaders within the same battle, bringing in fresh and rested ones to take the place of those who were beginning to get sluggish.