here is something else that just came to me. I remember, as a kid, seeing some people use " Santa exists so that proves Christmas and Christianity is the legitimate religion/holiday"
By obligating Teachers to keep such a pact/lie that parents present to their children, regardless of the religion other children and the teacher is, wouldn't it be obligating the teacher to support one religion over another?
I had seen a teacher put on the spot when a Christian and none Christian child argued over Santa. One child said that Santa didn't visit the other child because he was pagan and therefor bad. The poor teacher didn't know what to do. I do not remember how she settled the dispute. I just remember the look on her face. It was teh sort of wide eyed look horses get when startled.
That memory may be why the whole telling kids Santa is real makes me uncomfortable. In such a diverse world, someone is going to slip up, even if it means defending their own religion, and someone is going to get mad for spilling the beans to their child.