There is no reason for Santa to be mentioned at all. No reason to debate the belief in him what so ever. So, it doesn't really matter what YOU feel about 7 year olds or 12 year olds. The simple solution is not to discuss it. This teacher should have know that.
There are many cats that a teacher has no business "letting out of the bag".
BTW, 7 is hardly the age when they stop believing. For many kids this is during the age group that they are the most excited about his visit. They tend to no longer have any fear of him and are able to enjoy seeing him, writing him letters (something they are now able to do themselves), talking to him, and the excitement of Christmas eve and putting out cooking and milk.
WORD!
My daughter turned seven the beginning of Oct and her and her classmates in the first grade very much believe in Santa. And so what if they do? It is no one elses business or "job" to set them straight. I also have four children in their twenties and they believed up until about ten or eleven. God forbid we allow some magic or fun into the life of a child, the world might end.
