“What is a math teacher’s favorite sum?”/“Summer!”

A math riddle is:
   
Q: What is a math teacher’s favorite sum?
A: Summer.
 
That is, when there is no school (summer). The joke was posted on Twitter on February 29, 2012.
 
     
Twitter 
Kevin “K-Ship” Shipley
@Twisted_Steel16
What is a math teacher’s favorite sum?
Summer!
10:16 PM - 29 Feb 2012
 
Twitter
Jokes from my kid
@Jokesfrommykid
What is a math teacher’s favorite sum? Summer #jokes
9:15 AM - 1 Apr 2012
 
Twitter
Ashok Kushwaha
@ashok_1506
What is a math teacher’s favourite sum? Summer
3:14 AM - 21 Sep 2013
 
Google Books
Insults Anywhere Kids Presents
Hundreds Of Kids Jokes They Can Tell Anywhere!
By Franklin Yantz    
Insults Anywhere
2014
Pg. ?:
Q: What is a math teacher’s favorite sum?
A: Summer!
 
Google Books
School Jokes For Kids 5
By Peter Crumpton
PeteyRF Creative
2015
Pg. ?:
What is a math teacher’s favorite sum?
Summer!
   
Google Books
Have You Heard the One About . . .:
More Than 500 Side-Splitting Jokes!
By Justin Sedgwick
New York, NY: Skyhorse Publishing
2017
Pg. ?:
What is a math teacher’s favorite sum?
Summer!
 
Twitter
Alex Fowzer
@AFowLegacy
What is a math teacher’s favorite sum? Summer! #bigal #koyn939
9:14 AM - 29 Mar 2018