A War of Gifts (Hardcover) by Orson Scott Card

Orson Scott Card offers a Christmas gift to his millions of fans with A War of Gifts, a short novel set during Ender Wiggin's first years at the Battle School where it is forbidden to celebrate religious holidays.
The children come from many nations, many religions; while they are being trained for war, religious conflict between them is not on the curriculum. But Dink Meeker, one of the older students, doesn't see it that way. He thinks that giving gifts isn't exactly a religious observation, and on Sinterklaas Day he tucks a present into another student's shoe.

This small act of rebellion sets off a battle royal between the students and the staff, but some surprising alliances form when Ender comes up against a new student, Zeck Morgan. The War over Santa Claus will force everyone to make a choice.

THE ENDER UNIVERSE

Ender series
Ender’s Game / Speaker for the Dead / Xenocide / Children of the Mind / Ender in Exile / Children of the Fleet

Ender’s Shadow series
Ender’s Shadow / Shadow of the Hegemon / Shadow Puppets / Shadow of the Giant / Shadows in Flight

The First Formic War (with Aaron Johnston)
Earth Unaware / Earth Afire / Earth Awakens

The Second Formic War (with Aaron Johnston)
The Swarm / The Hive

Ender novellas
A War of Gifts / First Meetings