About 10 minutes into first period, when all the kids were silently working, the feeder crickets in the back started to chirp. It was so perfectly timed that at first I thought a kid was playing a sound effect from their phone. It was pretty funny.
Twenty minutes later a chair (which had been put up over night) fell off an unused table in the back. It was very loud, and very unexpected. After a few startled screams, most of the kids started laughing, and saying that the classroom is haunted. A few others started saying that it was a bad sign, and that I should cancel the test.
Later in the day, one of the class periods had already taken their exam, so we watched an episode of the BBC documentary series, Planet Earth.
The segment below really freaked the kids out.