Perhaps it is because Advent and Christmas are separate liturgical seasons and the school wishes to maintain that distinction for the children. But you will need to ask your children’s teachers to be certain. Depending on your children’s winter break schedule, you might suggest to your children’s teachers that they have a Christmas party for the children either on Twelfth Night (January 5) or on Epiphany (January 6), which are celebrated in Catholic countries as Little Christmas. In 2009, January 5 and January 6 will fall on a Monday and Tuesday.
As to the general question of Christmas parties during Advent, there is nothing that prohibits it, and sometimes (depending on individual circumstances) such an anticipatory celebration of Christmas is reasonable.
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