To “drop out” of school means to leave school. There are several skydiving school jokes.
   
“What kind of school do you have to drop out to graduate? Sky-diving school” was printed in the Tampa (FL) Tribune on October 4, 1993. “Casey: In what school do you have to drop out before you graduate? Lenni: Skydiving school!” was printed in the book The Laugh Rally (1995)  by Christina Wilsdon. “What school do you have to drop out of in order to graduate? Skydiving school” was printed in The Courier-Journal (Louisville, KY) on December 27, 1998.
   
“Skydiving school is the only school where you have to drop out to graduate” was posted on Twitter by Bradley Klein on April 25, 2011. “Skydiving school is the only school where you have to drop out to graduate” was posted on Twitter by ⓝ⑧ⓦⓞⓞⓓ④ⓓ on April 9, 2015.
 
       
Newspapers.com
4 October 1993, Tampa (FL) Tribune, “KidsLife,” pg. 6-Baylife, col. 4:
What kind of school do you have to drop out to graduate?
Sky-diving school.
   
Google Books
The Laugh Rally
By Christina Wilsdon
New York, NY: Bantam Books
1995
Pg. 14:
Casey: In what school do you have to drop out before you graduate?
Lenni: Skydiving school!
 
Newspapers.com
27 December 1998, The Courier-Journal (Louisville, KY), “Mini Ha-has,” pg. ?, col. 1:
What school do you have to drop out of in order to graduate?
Skydiving school.
 
Newspapers.com
1 April 1999, The Sun Times (Owen Sound, ON), Grey-Bruce This Week, pg. 6, col. 1:
GRIP ‘N GROAN
From which school do you have to drop out before you can graduate?
Skydiving school.
 
Google BOoks
April 1999, Boys’ Life, “Think & Grin,” pg. 61, col. 3:
Paul: Which school do you have to drop out of to graduate?
Raul: Which one?
Paul: Sky-diving school.
—Bradley Bernard, Louisville, Ky.
     
Google Books
School Daze:
A Book of Riddles about School
By Michael Dahl
Minneapolis, MN: Picture Window Books
2004
Pg. 16:
In what school do you have to drop out to graduate?
Sky-diving school.
 
Newspapers.com
2 April 2008, News Journal (Mansfield, OH), “Jumble” by Mike Argirion and Jeff Knurek, pg. 9B, col. 3:
What he turned into when he went to skydiving school—A DROP OUT
 
Twitter
Bradley Klein
@oncloudklein
Skydiving school is the only school where you have to drop out to graduate
@SpacemanQuisp @funnyoneliners
3:04 PM · Apr 25, 2011·Twitter for Android
   
Twitter
Bradley Klein
@oncloudklein
@funnyhumour Skydiving school is the only school where you have to drop out to graduate
4:24 PM · Apr 27, 2011·Twitter Web Client
   
Twitter
ZOO Thursdays
@DropNCL
For what school do you have to drop-out inorder to graduate?
Skydiving school.
11:03 AM · May 30, 2012·Twitter Web Client
     
Twitter
Joobaalee
@joobaalee
What is the only school were you need to drop out to graduate?
Skydiving school!
12:21 PM · Jul 22, 2013·Buffer
   
Twitter
Moon’s Daughter
@Justustalking2
School Joke…
Q: What’s the only school where you have to drop out to graduate?
A: Skydiving school
Blessings to all students #Raiseup
8:47 AM · Sep 4, 2014·Twitter Web Client
   
Twitter
ⓝ⑧ⓦⓞⓞⓓ④ⓓ
@N8wood4d
Skydiving school is the only school where you have to drop out to graduate.
1:02 PM · Apr 9, 2015·Twitter Web Client
   
Twitter
purpletrice
@martrice_cpa
Joke/Pun of the day:
At what school do you have to drop out in order to graduate?
Skydiving school.
9:42 AM · Jul 18, 2017·Twitter for Android
 
Reddit—Jokes
Posted by u/SoaringMutt March 19, 2019
I wasn’t cut out for skydiving school
So I dropped out
   
Twitter     
chloe hood🤠
@hoodchloe4
Replying to @VilchisBrixey
Went to skydiving school, had to drop out to graduate.
11:22 PM · Aug 3, 2020·Twitter for iPhone
 
Twitter
Avi Woolf, Wilderness Conservative🐺
@AviWoolf
What type of school requires you to drop out to graduate?
Skydiving school.
9:21 PM · Oct 26, 2021·Twitter for Android
 
Twitter
Funny Teacher
@SchoolJokesK_12
What school requires you to drop out in order to graduate?... #Skydiving school. https://bit.ly/3xkWPXQ #BackToSchool #teaching #students #1stdayofschool
6:01 PM · Aug 24, 2022·Twitter Web App
 
Facebook
Indian Hills Community Sign
November 15, 2022 at 1:20 PM ·
YOU MUST
DROP OUT
TO GRADUATE
FROM SKYDIVING
SCHOOL