That's one way of efficiently dispersing any annoyance whatsoever. John snorts, along with a short giggle, because Sherlock's never looked more ridiculous than in that moment.

"All right," he relents, and he supposes it is funny. No other reason for people to do it, really.

And now that Sherlock's had his amusement, he sucks in another breath for the sake of it, sends Sherlock a bit of a skew smile and walks into the classroom. Silent and deserted, as expected. Doubtful there's anything cursed. Might jump out and surprise them later, he supposes, but probably not.