“Why did the teacher write on the window?”/“Because she wanted the lesson to be very clear.”

A school riddle is:
 
Q: Why did the teacher write on the window?
A: Because she wanted the lesson to be very clear.
 
“Why did the teacher write on the window? He wanted his lesson to be clear” was printed in the Los Angeles (CA) Times on April 3, 2000. “Why did the teacher write on the window?  Because he wanted his lesson to be a clear one” was posted on the newsgroup alt.teens.penpals on April 11, 2000.
 
       
3 April 2000, Los Angeles (CA) Times, “The Kids’ Reading Room; Jokes & Riddles,” pg. E8:
Why did the teacher write on the window? He wanted his lesson to be clear. (Jiamiao, 8, Westpark Elementary, Irvine)
 
Google Groups: alt.teens.penpals
Directions.
merry mcgrath
4/11/00
(...)
3.  Why did the teacher write on the window?  Because he wanted his lesson to be a clear one. 
   
Google Books
Joke & Riddle Ballyhoo
By Jacqueline Horsfall
New York, NY: Sterling Publishing Company, Inc.
2005
Pg. 61:
Why did the teacher write on the window?
He wanted the lessons to be really clear.
 
Twitter
99.5 The Wolf‏
@995thewolfdfw
Exclusively for the kids today on this First Day of School:
Q: Why did the teacher write on the window?
A:... http://fb.me/2gYaEBKV6
6:54 AM - 27 Aug 2012
 
Twitter
FabulousSavings.CA‏
@CanadaCoupons
Why did the teacher write on the window? To make the lesson very clear! #LunchtimeFunnies
12:10 PM - 6 Sep 2012
 
Twitter
Craig Baillie‏
@Baillie1985
@laurenharding14 why did the teacher write on the window?? ......... cos she wanted the lessons 2 b very clear! Hahaa OK I’m done!.........
5:26 PM - 9 Sep 2012
       
Google Books
School Jokes For Kids 5
By Peter Crumpton
PeteyRF Creative
2015
Pg. ?:
Why did the teacher write on the window?
Because she wanted the lesson to be very clear!
 
3 January 2015, Newsday (Long Island, NY), “Kidsday,” pg. B21:
Why did the teacher write on the window?
He wanted his lesson to be very clear.
- Anthony Friscia
 
Twitter
Kidstaff‏
@Kidstaff
#tuesdaychuckle Why did the teacher write on the window? Because she wanted the lesson to be very clear! #haha #jokeoftheday #teacherjoke
2:15 AM - 5 Dec 2017