Chapter 2: peter, page 12. Peter hates Ender because he had his monitor for a year longer than him. Peter is a bit crazy and threatens to kill Ender while stepping on his chest. He eventually lets up on Ender but threatens him by saying someday he will kill him.
Commander
Chapter 1:third, page 8. Stilson was bullying Ender because without Ender's monitor the people wouldn't rescue him. The kids kept bullying Ender until he kicked stiltson in the ribs and knocked him down. He then proceeded to kick stilton over and over again and then run to the bus. we discover later in the book that Stilston actually died from his injuries.
Bonzo
Chapter 6: The Giant's Drink, Page 65. Ender plays a game on his desk where he had to get past a giant as a mouse. He kept dying to the drinking game which was rigged to kill him every time. so instead of drinking the liquid he jumped on the giants face and burrowed into the giant's eye until it died.
Ender's teacher
Chapter 10: Dragon, page 156. This is when Ender became the commander of the dragon army. The dragon army was a new army Ender had never heard of before. He found this out when Anderson let him into his private to get him into his new flash suit.
Chapter 12: Bonzo, pg 211. Bonzo came too attack Ender in the bathroom. They fought each other and Ender won. Later in the book we found out that Ender killed Bonzo in the bathroom.
Chapter 14: Ender's teacher, Page 295.  Ender thinking that this is his final exam against his teacher loads up the game. He finds out the enemies outnumber him a thousand to one. He gives up and just decides to fly towards the planet and blows it up. We then discover that it wasn't a game at all and he just saved the world and defeated all of the buggers.