Mr Fyde

Morty Fyde is the former science teacher at Jerome Horowitz Elementary School.

Attack of the Talking Toilets
Although Mr. Fyde was not shown in the book, he was eaten by the Talking Toilets like the rest of the faculty. He was later regurgitated.

Invasion of the Incredibly Naughty Cafeteria Ladies From Outer Space (and the Subsequent Assault of the Equally Evil Lunchroom Zombie Nerds)
Mr. Fyde was shown for the first time in the third book. He is heckled by George and Harold, who make animal noises to make him think he is going nuts. He shows a baking soda volcano to the class before he goes off to see a doctor, afraid he is losing his mind. Like the rest of the school, he is turned into a zombie nerd, but is reverted by George and Harold.

Perilous Plot of Professor Poopypants
After George and Harold rig the teachers lounge, Mr. Fyde witnesses the teachers chasing after George and Harold, but thinks they are abominable snowmen (they were really just covered in paste and styrofoam packaging peanuts). The next day, Mr. Fyde retires, thinking he has gone insane, and leaves to the greener pastures of "The Piqua Valley Home for the Reality-Challenged".

Character Description
Although he is not cruel to children like the rest of the teachers at the school, Mr. Fyde is dorky, boring, and rather dim-witted. After George and Harold's antics make him think he is insane, he retires.

Trivia

 * His name is a pun on the word "mystified", while his full name is a pun on the word "mortified".