When Timmy wakes up his parents on the first "repeat Christmas," they think that Timmy has to go to school and they have to go to work. Timmy wouldn't be going back to school until after New Years', even if it wasn't Christmas every day.
In this episode, school is only closed on Christmas, but in real life, Christmas break is two weeks.
The previous episode "Dream Goat" establishes that if Timmy wishes for something, Cosmo and Wanda have to grant it immediately unless it would violate one of the fairy rules. Here, during a musical number, Timmy wishes for it to be Christmas every day multiple times, yet the wish is not granted until after the musical number, and Cosmo even hesitates.