A joke is told about a parent reading a report card:
 
FATHER/MOTHER: Johnny, why did you get such low grades after Thanksgiving/Christmas?
JOHNNY: Everything is marked down after the holidays.
 
The joke (in slightly different form) was printed in the New York City humor magazine Puck on January 28, 1903.
 
 
28 January 1903, Puck (New York, NY), pg. 10, col. 1:
ACCOUNTED FOR.
MOTHER.—Johnny, how is it you stand so much lower in your studies in January than you did in December?
SON.—Oh! Everything is marked down after the holidays, you know, Mother!
 
29 June 1929, Chicago (IL), Defender, “Humor,” pt. 2, pg. 3, col. 6:
A REAL ALIBI
Father—Why are your grades so low since the holidays?
Son—You know everything is marked down after Christmas.
—Miss Rose Lee Grace, 408 Mercer St., Albany, Ga.
 
13 January 1935, Illinois State Journal (Springfield, IL), “The Honor Roll,” The Junior Journal, pg. 4, col. 4:
Mother: “Why is it, Jimmy, that you get lower grades in January and February than in December?”
Jimmy: Because everything is marked down after Christmas.”
—Velma Byers, Chatham.
 
21 February 1947, Illinois State Journal (Springfield, IL), pg. 6, col. 3:
Dad scowled over his son’s report card. ‘Why are your grades so much lower for January than December,” he demanded.
 
“I’m surprised at you, dad,” said the son in an aggrieved tone. “Don’t you know that everything is marked down after the holidays?”
 
5 February 1950, The Times-Picayune New Orleans States (New Orleans, LA), pg. ?, col. 3:
‘January F’ Sale
LITTLE TOMMY’S father wasn’t very pleased with last month;s report card. “How is it,” his father asked, “that you did so much worse in January than you did in December?”
 
“Why, Daddy,” Tommy replied in injured tones, “don’t you know that everything is marked down after the holidays?”—Mrs. R. W., 3713 St. Claude.
 
Google Books
February 1959, Boys’ Life, “Think and Grin,” pg. 78, col. 1:
THe Mother took one look at her son’s report card and exclaimed, “My goodness, Johnny, this is certainly terrible. Can you explain what happened?”
 
“You know how it is, Mom. Things are always marked down after the holidays.” — Martin Eades, Geneva, N. Y.
 
21 November 2001, The Star Democrat (Easton, MD),  “Thanksgiving Riddles and Jokes,” pg. 8A, col. 2:
Why did Johnny get such low grades after Thanksgiving?
Because everything is marked down after the holidays.
 
Google Groups: alt.humor.puns
Puns of the Weak 02-06-04 Part 2 The Puns
Stan Kegel
2/6/04
(...)
“It’s not good, Joe. Your grades have taken a nose dive since the holidays.” “I know. Dad. But there’s a good explanation!” “Oh, Yeah?” “Yeah, You know how everything gets marked down after Christmas!” (One Big Happy: Rick Detone)
   
November 2006, Professional Safety, “Turkey Day Clunkers,” Back Page, pg. 64, col. 2:
Why did Johnny get such low grades after Thanksgiving?
Because everything is marked down after the holidays.
 
Twitter
OCAEC
‏@OCAEC
Thanksgiving literature (?):  Why did Johnny get such low grades after Thanksgiving?  Because everything is marked down after the holidays!
8:06 AM - 18 Nov 2009
   
November 2010, National Geographic Kids, “Just Joking,” pg. 3:
Q Why did Johnny get such low grades after Thanksgiving?
A Because everything is marked down after the holidays.
 
Google Books
The Happy Classroom:
From Ha-Ha to A-Ha
By Janet Aaker Smith
Marion, IL: Pieces of Learning
2011
Pg. 87:
Why did Johnny get such low grades after Thanksgiving?
Because everything is marked down after the holidays.
 
Google Groups: The Grif.net
12/09/11 Grif.Net - Christmas Q&A
Dr. Bob
12/9/11
(...)
Why will you get poor grades after the Christmas holidays?
Because everything is marked down.