Then don't lie to your child. Children are full of imagination, but they shouldn't be taught to believe in a particular person if you don't believe in it to start with.
You teach your child to not lie, yet you lie to your child. Hence the term, hypocrisy. The teacher made a mistake in sharing the truth, but not because it was the truth, it was because of how irrational the parents may become.
Children should be able to enjoy the stories of Santa Claus, just like they enjoy the stories of Harry Potter. The magic is there, and it exists with the stories of Santa Claus. Yet they know Harry Potter does not exist in the real world.
Why should an exception be made with Santa Claus? He is a part of the popular Christmas folklore. The fact is, people treat him as if he's more real than Jesus Christ himself.
Saint Nicholas existed centuries ago, but he is long dead. His spirit is still in our hearts and minds. Corporations like Coca-Cola popularized his legacy and turned it into a commercialized blitz. The monster gets bigger every year to the point where it's completely devoid of any true spirits of Christmas.
I still celebrate Christmas the way I used to as a child. It's by getting together with family and having a feast. We do exchange a few gifts, but that's not my favourite part of Christmas. It's being with my family that is my favourite part. Celebrating Christmas is a way of reaffirming my kindness and love for the others and the life that we have come to cherish that I practice all year long.