A popular college saying is:
 
Q: What’s the difference between a college freshman and a high school senior?
A: Three months of summer vacation.
 
That is, there is not much time to prepare for college. “Between the high school senior and the college freshman there is only three months’ difference—and three vacation months at that” was published in the Cooperative School Bulletin in 1931.
 
                   
Google Books
Cooperative School Bulletin
Volume 2, Issue 6
1931
Pg. 10:
Between the high school senior and the college freshman there is only three months’ difference—and three vacation months at that.
 
Google Books
Proceedings of the ... Annual Meeting of the American Association of Collegiate Schools of Business
Issue 19
Ronald Press Company
1937
Pg. 61:
I have taught in both high school and college and I know that the difference between a high school senior and a college freshman is just a matter of three months.
 
Google Books
Social Science (Pi Gamma Mu)
Volume 13
1938  
Pg. 206:
I have taught in both high school and college, and I know that the difference between a high school senior and a college freshman is just a matter of three months.
 
22 January 1961, The State Journal (Lansing, MI), “Michigan Schools Report: College Closing Doors To Marginal Students” by Stuart Gross, pg. 10, col. 7:
“The difference between a high school senior and a college freshman is only three months and fortunate is that student who enters college on the upbeat following a challenging senior year.”
(Marvin F. Pahl, registrar at Albion College.—ed.)
     
Google Books
U.S. News & World Report
Volume 69, Issues 18-26
1970
Pg. 49:
“And what is the difference, really, between a high-school senior and a college freshman? About three months.”
 
25 November 1983, Lexington (KY) Herald-Leader, “Berea College offers program to aid high schools” by Cheryl Truman, pg. B1, cols. 2-3:
Jackie Betts, a noted program coordinator, said, “The difference between a college freshman and a high school senior is three months. That’s not a lot of time.”
 
Google Books
Rise of the Phoenix
By Kenneth Whetstone
Genesis Press Incorporated
2006  
Pg. 209:
“How much difference is there between a high school senior and a college freshman? About ten months, that’s all.”
   
Twitter
Kyle Kingsolver‏
@kylekingsolver
my teacher says the joke wats the difference between a high school senior and a college freshman? 2 1/2 months…
11:33 PM - 13 Jan 2010
 
Twitter
David Stubbs‏
@drstubbs
@ArticlesEngine The difference between being a high school senior and a college freshman is 3 months…think about it.
10:21 PM - 17 Dec 2010 from Keystone, IA
   
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Short Stroke Messiah‏
@aGotDamnPHool
Wats the difference between fucking a high school senior and a college freshman? 2 weeks
7:06 PM - 20 May 2011
 
Twitter 
Noah‏
@nohbro
What’s the difference between a high school senior and a college freshman? 3 months.
7:45 AM - 7 Apr 2015
   
Twitter
me‏
@not_actuallyme
What’s the difference between a High School senior and a college freshman? via /r/Jokes http://ift.tt/2p897nj
3:56 PM - 16 Apr 2017
 
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Ivory Miles Williams‏
@miles_ivory
The difference between a high school senior and a college freshman is (3) months…
3:26 PM - 4 Jun 2017
 
San Francisco (CA) Chronicle
We must teach our kids how to disagree
By Jonathan Zimmerman September 25, 2017 Updated: September 25, 2017 4:26pm
Here’s an old joke that you hear every September if you teach at an American university as I do. It’s not that funny, really, but you’ll get the point.
 
Q: What’s the difference between a college freshman and a high school senior?
A: Three months of summer vacation.