"I went to school to become a wit -- only got…

A half-wit is a foolish or imbecilic person. “I went to school to become a wit—only got halfway through” is a jocular saying that has been cited in print since at least 1999 and is of unknown authorship.

Merriam-Webster Dictionary
half–wit noun \ˈhaf-ˌwit, ˈhäf-\
Definition of HALF-WIT
: a foolish or imbecilic person
First Known Use of HALF-WIT
1640

30 March 1999, Rocky Mountain News (Denver, CO), “Spotlite”:
I went to school to become a wit, and only got halfway through.

Google News Archive
9 September 1999, Western Kansas World (Trego County, KS), “Wisdom of Life,” pg. 13, col. 6:
1 went to school to become a wit; only got halfway through.

11 September 1999, Daily News-Record (Harrisonburg, VA), “Bishop’Mantle” by Jim Bishop, pg. 19, cols. 5-6:
I went to school to become a wit—only got half way through.

Google Books
A Treasury of Humor:
Thousands of humorous stories, jokes, anecdotes and one-liners, up-to-date and categorized for ease of use
By Lowell D. Streiker
Peabody, MA: Hendrickson
2000
Pg. 21:
I went to school to become a wit. I only got halfway through.

Google Books
It’s Better to Die Laughing Than to Be Dead Serious:
How to Be the Life of the Party, the Podium and Everywhere in Between
By Marvin Maupin
Bloomington, IN: Authorhouse
2010
Pg. 47:
“I went to school to become a wit, only got halfway through.”