You Guys Already Knew, Right?
Kids are extra susceptible to peer pressure, and so if most of the class does or says something, the rest of the kids would want to act the same way, to avoid being labeled as outcasts. When you’re a kid, you never want to be different or stand out, but you want to blend in. Some people never lose that and still strive to blend in and act like everyone else, even as adults.
So, when this teacher let it slip that Santa wasn’t real, the entire class acted as if they already knew. I’m willing to bet there was more than one kid who was shocked by this new information – but they all acted as if they weren’t phased by it because that’s how the majority of the class acted.