“Why didn’t the skeleton want to go to school?”/“Because his heart wasn’t in it.”

A skeleton riddle is:
 
Q: Why didn’t the skeleton want to go to school?
A: Because his heart wasn’t in it.
 
The Halloween joke has been cited in print since at least 1987.
 
     
Google Books
Holiday Entertaining
By Jean Paré
Edmonton, Alberta: Company’s Coming Pub.
1987
Pg. 111:
The little skeleton didn’t want to go to school. His heart wasn’t in it.
 
Google Books
Funny You Should Ask:
How to Make Up Jokes and Riddles with Wordplay
By Marvin Terban
New York, NY: Clarion Books
1992
Pg. 58:
Why did the skeleton quit his job?
Because his heart wasn’t in it.
 
10 February 1992, Vancouver (BC) Sun, “There’s still time for fun in the snow,” pg. C6:
Q: Why didn’t the skeleton want to go to school?
A: Because his heart wasn’t in it.
 
2 May 1992, The Vidette-Messenger (Valparaiso, IN), “Mighty Funny’s Mini Jokes,” The Mini Page, pg. 1, col. 3:
Q: Why did the skeleton flunk?
A: Because his heart wasn’t in it.
(Sent in by Ingrid Tommassoni)
 
Google Books
Big Bad Classroom Jokes
By Sandy Ransford
London, UK: Robinson Children’s Books
1998
Pg. ?:
Why didn’t the skeleton do well at school?
His heart wasn’t in it
 
Google Groups: alt.humor.puns
[Fwd: Submit: Halloween Riddles]
Stan Kegel
10/30/00
(...)
Why didn’t the skeleton want to go to school??
Because his heart wasn’t in it. (Irene A. Mystery)
 
Google Books
School Jokes
By Joseph Rosenbloom
New York, NY: Sterling Publishing Company, Inc.
2003
Pg. 9:
Why did the little skeleton flunk out?
His heart wasn’t in it.
 
Google Books
The Super, Epic, Mega Joke Book for Kids
By Whee Winn
Grand Rapids, MI: Zonderkidz
2016
Pg. ?:
Why didn’t the skeleton want to go to school?
His heart wasn’t in it
 
Twitter
Team Propeller‏
@propeller_learn
Why didn’t the skeleton want to go to school?
His heart wasn’t in it! 💀 #HappyHalloween
3:15 AM - 31 Oct 2017