When I still worked for a private school, I injured myself one weekend and came to work on Monday with a black eye.
It was fake injury day, so I blended right in.
After a couple of days the kids figured out it was real and started asking how it happened. Because kids are totally oblivious to everything around them, I had fun making up a different story for each kid that asked.
The kids were oblivious enough that they rarely noticed when I told conflicting stories in their presence. It took them at least 6 months to sort it out, and as far as I'm aware several of them still believe there was a polar bear, plane crash, and/or ostrich involved.