“True terror is to wake up one morning and discover that your high school class is running the country” is a quotation that has been credited to American author Kurt Vonnegut (1922-2007). The quotation has been cited in print since at least 1984, but the exact source hasn’t been found.
 
   
Wikipedia: Kurt Vonnegut
Kurt Vonnegut, Jr. (/ˈvɒnɨɡət/; November 11, 1922 – April 11, 2007) was an American writer. His works such as Cat’s Cradle (1963), Slaughterhouse-Five (1969), and Breakfast of Champions (1973) blend satire, gallows humor, and science fiction. As a citizen he was a lifelong supporter of the American Civil Liberties Union and a critical pacifist intellectual. He was known for his humanist beliefs and was honorary president of the American Humanist Association.
 
Google News Archive 
27 June 1984, Windsor (Ontario) Star, “Spain’s PM loses idealism” by John Darnton (N. Y. Times Service), pg. A6, col. 5:
For many Spaniards who came of age during the puritanism of Franco, it was fie shock of waking up one day and finding, in the phrase of Kurt Vonnegut that your high school class is running the country.
   

Google Books
Reader’s Digest Condensed Books
Volume 3
By Reader’s Digest Association.
Pleasantville, NY: Reader’s Digest Association
1986
Pg. 427:
“You remind me of my youth. Hey, I read this somewhere. You want to know what’s the really scary thing about middle age?” She nods. “It’s waking up one morning,” he tells her, “and realizing that your high school class is running the country.”
 
GOogle Books
Trust Your Mother, But Cut the Cards
By Sidney Zion
Fort Lee, NJ: Barricade Books; Emeryville, CA: Distributed by Publishers Group West
1993
Pg. 8:
Kurt Vonnegut wrote: “True terror is to wake up one morning and discover that your high school class is running the country.”
 
Google Books
Teachers: Jokes, Quotes, and Anecdotes
By LLC Andrews McMeel Publishing
Kansas City, MO: Andrews McMeel Publishing
2008
Pg. 33:
True terror is to wake up one morning and discover that your high school class is running the country.
Kurt Vonnegut Jr.