Chapter 10: Dragon
3. Why did Graff give Ender all new soldiers he had no worked with before?
Graff gave Ender's new Dragon army new soldiers consisting of mostly untrained Launchies to see whether or not he is capable of leading a team with no previous experience, which is what he will have to do once he saves the world.
6. What rules are changed, why do you think the teachers changed them?
1. No trades allowed.
2. No informal training sessions.
And many more!!
The teachers keep changing the rules because they know when war comes, the enemy won't settle for a dispute on whether or not the other team is ready, they will just attack.