If you children are bad," she warned, "or if you answer a problem wrong, I'll wiggle my ears, stick out my tongue, and turn you into apples! (1.2)
Okay, so this doesn't really sound like a normal teacher, does it? Here's the first clue in Sideways Stories that the supernatural is actually a real thing at this school.
Mrs. Jewls didn't stop. She took off another one of his coats, and another. Sammy was only four inches tall, three inches tall, two inches tall. At last she removed the final coat. All that was there was a dead rat. (14.36-37)
This is almost heading into Stephen King territory, isn't it? If a dead rat can walk around in layers of smelly raincoats, some kind of weird magic is at work.
Suddenly she screamed. The chalk turned into a squiggling worm! She dropped it on her foot. (29.21)
Chalk that turns into a worm? Yep—we're definitely in the realm of the supernatural.
A woman appeared on the screen. She had a long tongue and pointed ears. She stepped off the screen and into the classroom. It was the ghost of Mrs. Gorf. (29.23-24)
Eek, right? But don't worry—if she ever comes back, just try to hug her and she'll quickly disappear.