A schoolteacher pun about “pupils” is:
 
Q: Why did the cross-eyed teacher loser her job?
A: She couldn’t control her pupils.
 
The joke has been cited in print since at least 1942 and has been printed on several images.
 
“Why did the Cyclops give up teaching?”/“He only had one pupil” is a similar joke.
 
   
25 August 1942, The State Journal (Lansing, MI), “Looking Over Lansing” by Walt, pg. 5, col. 2:
Then there was the school teacher with eye trouble. She couldn’t control her pupils.
 
11 January 1952, The Daily Republic (Mitchell, SD), “Chuckles,” pg. 4, col. 3:
Bennett Cerf: Russell B. Hopkins is preoccupied with stories about cross-eyed citizens. His local board of education, he tells me, discharged a cross-eyed teacher because they couldn’t see eye to eye with her and she couldn’t control her pupils.
 
Google Books
Attitudes of Speech Defectives Toward Humor Based on Speech Defects
By Leola Schaper Horowitz
Ph.D. thesis
Department of Speech and Drama, Stanford University
1954
Pg. 104:
A beard of education discharged a cross-eyed teacher because they couldn’t see eye to eye with her and she couldn’t control her pupils.
 
4 December 1963, Philadelphia (PA) Inquirer, Herb Stein column, pg. 52, col. 6:
Down at Romaine’s Mexican Restaurant, not all the corn goes into tortillas. Owner Cliff Gates is telling customers about the cross-eyed school teacher. She couldn’t control her pupils. Frank Rhoades)
   
Google Books
February 1967, Boys’ Life, “Think and Grin,” pg. 70, col. 1:
Len: Why did the cross-eyed teacher lose her job?
Ken: Because she couldn’t control her pupils. — Mark Stein, West Kingston, R.I.
 
13 July 1993, Chicago (IL) Tribune, pg. F11, right masthead:
Why did the cross-eyed teacher lose her job?
She couldn’t control her pupils.
 
10 August 1996, The Post and Courier (Charleston, SD), “Meet the folks who don’t want easy answers” by Elsa McDowell, pg. 1:
Have you heard about the cross-eyed teacher who was fired because she couldn’t control her pupils?
     
5 January 1998, Los Angeles (CA) Times, “Laugh Lines: Pint-Sized Punch Lines,” pg. E4, col. 3:
Did you hear about the cross-eyed teacher? She couldn’t control her pupils. (Garrett Asanuma, 11, Simi Valley, White Oak Elementary)
 
Twitter
Michael Klobe‏
@klobetime
There was once a cross-eyed teacher who couldn’t control her pupils.
12:16 PM - 5 Aug 2008
 
Twitter
Liezell‏
@liezell
why was the teacher cross-eyed?
She couldn’t control her pupils.
11:22 PM - 15 Nov 2008
 
Google Books
School Jokes For Kids
By Peter Crumpton
PeteyRF Creative
2015
Pg. ?:
Why did the crosseyed teacher lose her job?
Because she couldn’t control her pupils.
   
Google Books
The Super, Epic, Mega Joke Book for Kids
By Whee Winn
Grand Rapids, MI: Zonderkidz
2016
Pg. ?:
Why were the teacher’s eyes crossed?
She couldn’t control her pupils.
   
Twitter
Excuse the Pun‏
@ExcuseThePun
Why did the cross-eyed teacher lose her job?
Because she couldn’t control her pupils.
11:29 AM - 22 Dec 2016