Had Sherlock been anyone other than Sherlock, John might have somewhat suspected something like that. It's really rather typical when thinking about it, especially if someone's a little tense about going into a possibly cursed classroom. But the fact that he doesn't think it's cursed probably just adds to the leap of his heart - a very brief moment of fright before he realises that Sherlock just. Apparently felt like it? Which is really unexpected.

He sucks in a breath, takes just a moment to gather himself together again, and finally sends Sherlock a look for his efforts. Hint of disapproval? Definitely. But mostly some dry kind of amusement, and a bit of disbelief.

"Sherlock. What?"