I became a somewhat involved spectator in a similar incident.

I became a somewhat involved spectator in a similar incident...
The biology teacher at my high school, Mr. Evans, was an incurable wit. He
was the one teacher everybody liked. He was the one who made sure that we
dissected Ascaris worms (long white stomach worms) the same day the lunch
room served spaghetti. One day, he fished out a four-foot preserved boa
constrictor and laid it on the floor just inside the biology lab door. Then
he put a preserved frog in its mouth. Then he stood by the door waiting for
class to start, watching students' reactions as they opened the door. I had
the misfortune to arrive right behind one of the more excitable girls.
(click.) (door opens) AAAAAAAAAAAAK! She ran right over me!

Mr. Evans related tales of his college days. He said one of his professors
was a real joker (by HIS standards!) who let his pet tarantula roam loose in
the room during class. You could track its progress by watching people pick
up their feet. He made some ammonium tri-iodide and painted it on the floor
before class. People walk in. BANG! POP! POW! When you pick up one foot, you
have to put the other one down. BAM!

I always wanted to put some inside the school bell. Ding-BOOM!