A school riddle is:
 
Q: What do you call the head of a school of fish?
A: A Sardean. 
 
“Sardean” is not a “dean,” but a pun on “sardine.” Forms of the “Sardean” joke have been cited in print in 1865 and 1996, on X/Twitter in 2012 and 2013,  and on Reddit—Jokes in 2021.
 
     
Google Books 
26 August 1865, The Family Herald, pg. 272, col. 1:
“That was a horrible affair,” said a gentleman in company, “the murder of Dean, and the sealing up o his remains in a tin box!” “What Dean?” asked half a dozen voices at once. “Sar Dean,” replied the wag.
 
Google Groups: alt.sex.fetish.tickling
“The Dating Game”
MstrsLaVon
Oct 13, 1996, 3:00:00 AM
The Dating Game
Entry Number Two
By Mistress LaVon
(...)
You just… ummm. you
reminded me of a joke.” said Sandy as she grabbed Marks arm to keep him in
his seat. Sandy was breathing hard and almost in tears. Of course Mark
thought it was because he had threatened to leave so his head began to
swell once more. No smile found his face however.
“Okay, tell me!” he order. Kay could hardly keep from laughing
herself from Sandy’s attempt to bail herself out.
“Okay. Ummmm, what do… you call…. the head of ummm Oh what do you
call theeee head of a fish college.” said Sandy managing a fake smile.
“I dunno. What?” growled Mark.
“The Sar DEAN! Get it?” Hopelessness filled Sandy’s face as Mark just
glared
   
X/Twitter
rusty
@1badpup
if u were a fish an u were tryin to get into college ud have to ask the sardean of admission s
4:55 PM · Sep 27, 2012
 
X/Twitter
mangeese
@icantcycle
“Who runs a school of fish?” “The sar-dean.” guys i laughed way too hard for that one and the seal one
4:54 AM · Jun 26, 2013
     
X/Twitter
Gina Nagel
@blahblahblah237
“@typixal
who runs a school of fish? a sardean” oh my goodness the best joke ever.
7:18 PM · Jan 15, 2014
   
X/Twitter
Evan B. Jaynes
@BullCityBowler
If a group of fish is called a school, that means the leader is called the sarDEAN
7:23 PM · Jul 12, 2017
 
X/Twitter
Surely U Jest
@DrSurelyUJest
That reminds me. What leads a school of fish
A Sar-Dean
11:21 PM · Oct 22, 2019
     
X/Twitter
Destry
@DestryBrod
What do you call the leader in a school of fish?
The sardean.
10:36 PM · Mar 24, 2020
 
X/Twitter
Secret Thinkers
@SecretThinkers
*fish school*
“I’m going to tell the sar-Dean”
12:54 PM · Mar 8, 2021
 
Reddit—Jokes   
Posted by u/International_Bee653 December 14, 2021
What type of fish leads its school?
a sardean
 
Reddit—Dadjokes
Posted by u/TheMightyBattleSquid December 17, 2021
What do you call the head of a school of fish?
The Sardean.
     
Reddit—Dadjokes
Posted by u/Sytivas October 5, 2022
What do you call the leader of a school of fish?
The sardean
 
Reddit—Cleandadjokes
Posted by u/TheMightyBattleSquid April 8, 2023
What do you call the head of a school of fish?
A Sardean.
 
LaffGaff
Fish School Head
January 7, 2024 by laffgaff
What do you call the head of a school of fish?
A Sardean.