Orson Scott Card
The human race is at war with an insectlike alien race. The first battles went badly, and now Earth prepares to defend itself against the imminent threat of total destruction at the hands of an inscrutable enemy. All focus is on the development and training of generals who can fight such a war - and win.
The long distances of interstellar space have given hop to the defenders of Earth - they have time to train these future commanders from childhood, forging them into an irresistible force in the high-orbital facility called the Battle School.
At the Battle School, there is only one purpose, only one curriculum: the strategy and tactics of war. The children are drawn from all nations, all races, all religious. There is no room for cultural differences. No room for religious observances, and certainly no room for Santa Claus.
But the young warriors disagree. When one of them leaves a Sinterklaas Day gift in his best friend's shoe, that quiet act of rebellion becomes the first shot in a war of wills that the staff of the Battle School never bargained for.