My mom pretty much 'fessed up after we started asking too many questions about Milton Bradley and other brands on toys. I think we'd been suspicious for awhile before it was confirmed.
When I was in HS, I think Junior Year, Catholic HS, and one of the nuns was talking about something and used an analogy and said "of course Santa isn't real" and one of our comic students (I loved the guy, he was so funny) goes, "OH NO!" in a shocked voice - and for a brief moment, the nun fell for it, you should have seen her face.
"Fun must be always." - Tomas Hertl, San Jose Sharks
"It was fun." - Kirk, Star Trek: Generations