Why does Pol Slattery switch Armies after becoming a commander?

Question

When Ender first encounters Pol Slattery, he is in command of Leopard Army.

The enemy was Leopard Army. It would be easy. Leopard was new, and it was always in the bottom quarter in the standings. It had been organized only six months ago, with Pol Slattery as its commander. Ender put on his new battle suit and got into line; Bonzo pulled him roughly out of line and made him march at the end. You didn't need to do that, Ender said silently. You could have let me stay in line.

When Ender sees him three years later, however, he is in command of Badger Army.

They won, of course, but it was a grueling affair, with the battleroom so filled with a labyrinth of stars that hunting down the enemy during mop-up took forty-five minutes. It was Pol Slattery's Badger Army, and they refused to give up.

Why did he switch armies? Does that ever happen to commanders?

Also, based on the following, he would have probably been 11 when made a commander,

"Content. He's doing well. His mind is keen, his play is excellent. Young as he is. we've never had a boy better prepared for command. Usually they go at eleven. but at nine and a half he's top flight."

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Carn Carby came to Ender as soon as he was unfrozen. He was a twelve-year-old, who had apparently made commander only in his last year at the school. So he wasn't cocky, like the ones who made it at eleven.

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Ender watched from the corridor. Pol Slattery was young, but he was sharp, he had some new ideas.

which would make him something like fourteen or fifteen three and a half years later. What is he still doing in Battle School? Mick says that he was getting too old when he was just thirteen.

A bigger boy came to sit by him. Not just a little bigger- he looked to be twelve or thirteen. Getting his man's growth started.

"Hi," he said.

"Hi," Ender said.

"I'm Mick."

"Ender."

[more dialoge]

"Yeah, funny, but no joke. I got nowhere here. I'm getting big now. They're going to send me to my next school pretty soon. No way it'll be Tactical School for me. I've never been a leader, you see. Only the guys who get to be leaders have a shot at it."

Answer 1

The only commander we follow from start to finish is Ender, and it can safely be assumed that when he arrived, they threw out most of the rule book. So to say that no commander ever changed armies is simply not within our limited view of Battle School.

Even though Graff is very Ender centric, that doesn't mean Battle School isn't still trying to make good soldiers of everyone else.

Pol Slattery was young, but he was sharp, he had some new ideas.

It is possible that at some point the teachers said, lets mix things up for Slattery and switch his army and see if he can cope with a completely new group of kids and still perform at the same level.

It is also possible that because he seemed to be brighter and fresher than some of the other commanders, they delayed his gradation in order to allow Ender to have a battle with him. Similar to the Bonzo situation.