From Orson Scott Card Wiki

[edit] Spoiler warning: Plot and/or ending details follow.

In Card's fictional future universe, humanity is at war with an alien species known as the Formics, or, colloquially, buggers. Battle School is the training ground for the future leaders of Earth's war effort—most notably Ender Wiggin and Bean. At the school, they undergo a rigorous curriculum designed to teach military strategy and tactics. As befits an elite school, only the very best and brightest students are taken. Prospective students must undergo invasive monitoring, and a battery of tests, to see if they are suitable for the school.

Students are generally taken to Battle School at a very young age—5 or 6 years old. The common age of "graduation" is usually 12. While at Battle School, they are taught academic subjects, including especially a heavy grounding in mathematics and science, a setup for work in space; for instance, Card describes Ender as learning trigonometry, which in the United States is not taught until high school.

However, the academic work is not the focus of Battle School. Rather, the true evaluation of the students comes in battle simulations which are run by the school's instructors. The students are divided into 40-person armies, most named after an animal (real or mythological). They fight mock battles in weightlessness, armed with harmless lasers and suits that freeze when hit with a laser shot. These battles are used to test the tactical aptitude of the students. Their performance is used to evaluate their progress. As such, the student body places a great weight on the standings in these games, a perception which is encouraged by instructors. The object of the game is supposedly to 'destroy' or freeze all the enemy soldiers and then pass through the gate the enemy entered through, but unconventional tactics on Ender's part somewhat subverted this victory condition. To win the game, four soldiers must touch their helmets to each corner of the enemy gate, and one soldier must go through the gate.

Breakdown of Battle School Armies