“Where do children learn their ABCs?”/“In LMN-tary school”

A popular alphabet joke is:
 
Q: Where do children learn their ABCs?
A: In LMN-tary school.
 
The joke has been cited in print since at least 1996.
     
 
Google Books
Funniest Riddle Book in the World
By Morrie Gallant and Sanford Hoffman
New York, NY: Sterling Publishing Company, Inc.
1996
Pg. 14:
Where do children learn their ABCs?
In LMN-tary school.
 
Google Books
Over 1500 Fantastic Jokes
By Guy Campbell and Mark Devins
London: Egmont Books, Limited
2003
Pg. 215:
Where do children learn their ABC’s?
At LMN-tary school!
 
Klosi Jokes
Category: School jokes
Date Added: 11/10/2007
Joke: Where do children learn their ABCs ?At LMN-tary school !
 
Google Books
101 Short and Hilarious Jokes for Kids:
Jokes They’ll Tell Again and Again
By Scott Allen
Delabarre Publishing at Smashwords
2010
Pg. ?:
Where do children learn their ABCs?
At LMN-tary school.
 
Google Books
A Tribute to Teachers:
Wit and Wisdom, Information and Inspiration about Those Who Change Our Lives
By Richard Lederer
Portland, OR: Marion Street Press
2011
Pg. ?:
Students Learn their ABCs at LMN-tary School.
 
Twitter
989ers
‏@syhezawumebo
Where do children learn their ABCs? At LMNtary school! ok
6:16 AM - 6 Feb 2015
 
reddit
To learn the alphabet, you go to LMN-tary school.
submitted June 30, 2015 by Shigjetar