She's a teacher in a religious education class. She's teaching the curriculum that Christmas is about Jesus, not Santa.
Bottom line: if you want to instill and perpetuate a myth for as long as possible in your child's life, don't voluntarily send your child to a place where that myth may be brought into question. Especially not to a religious class if that myth is about a religious event or holiday.
But unless this church makes it explicit that its doctrine excludes Santa (not all churches do), it was inappropriate for the teacher to take this decision out of the parents' hands. As someone else pointed out, she could have easily stressed the *real* meaning of Christmas without getting Santa involved.