The phrase blackboard jungle denotes the education system regarded as a place where the law of the jungle applies.
—Cf. also asphalt jungle | concrete jungle.

The phrase blackboard jungle was coined in the sense of a school characterised by weak discipline and bad behaviour as the title of a 1954 novel by the American author Evan Hunter (1926-2005), born Salvatore Albert Lombino, who also used the pen name of Ed McBain.

This advertisement for Evan Hunter’s novel was published in The San Diego Union (San Diego, California) of Sunday 12^th June 1955:

The first novel to dramatize one of the top social problems of our day, “Blackboard Jungle” is a startling book which leaves the reader thinking of its implications for a long time! You will want to race through the author’s gripping plot of “juvenile delinquency.”

The phrase was subsequently popularised as the title of a 1955 cinema film adapted from the novel, directed by Richard Brooks (1912-92), and starring Glenn Ford (1916-2006) and Sidney Poitier (born 1927).

This advertisement for the motion picture was published in the Richmond Times-Dispatch (Richmond, Virginia) of Saturday 18^th June 1955:

FRIGHTENING!
CHALLENGING!
ABSORBING!
TEEN-AGE TERROR
“BLACKBOARD JUNGLE”

The plot of the novel, which remained virtually unchanged on screen, is as follows:

The Blackboard Jungle tells the story of Rick Dadier, an army veteran and recent college graduate who lands his first teaching job at an urban vocational high school called North Manual Trades. His all-male, racially diverse students are unmotivated and disrespectful, mockingly calling him “Daddy-O” on the first day of class and asking, “Hey, teach, you ever try to fight thirty-five guys at once?” One cynical older teacher tells Dadier that Manual Trades is the “garbage can of the education system,” and their job as teachers is simply to sit on top of the lid. Dadier’s idealism and mettle are quickly tested after he rescues a fellow teacher, Lois Hammond, from an attempted rape in the school library; he is subsequently attacked by a group of students upset by his heroics. The novice teacher struggles with thoughts of quitting, but he needs the job to support his pregnant wife. Moreover, he is determined to try to help, and in particular he reaches out to Gregory Miller (Sidney Poitier), one of his African American pupils who is considering dropping out. The final scene pits Dadier in a classroom fight against an armed student. To Rick’s surprise, the other students, lead by Miller, intervene and end up restraining the troublemaker by pinning him against the chalkboard with an American flagpole. This show of loyalty inspires Dadier, and he and Miller both agree not to give up on the school.
—from ‘They Turned a School Into a Jungle! How The Blackboard Jungle Redefined the Education Crisis in Postwar America’, by Adam Golub (California State University, Fullerton), published in ‘Film & History: An Interdisciplinary Journal of Film and Television’ (2009)

As Adam Golub also explains, throughout the spring and summer of 1955, the film was debated, denounced, banned, and scapegoated on account of its violent content and its sharp educational critique.

The phrase, therefore, gained currency very rapidly. The Hollywood columnist Dorothy Manners (1903-98) used it in the Columbus Evening Dispatch (Columbus, Ohio) of Tuesday 28^th June 1955:

A folksier, more wholesome story could not be found for filming than Ralph Moody’s “Little Britches”¹ which Norman [Taurog²] will direct and produce for his independent Tau-Ream Productions right after the first of the year.
“Little Britches,” as millions of readers in 20 languages know, is the story of a boy, his rancher dad and their neighbors—such good family stuff it is guaranteed to have one of two effects on children:
It could drive the juvenile delinquents back to the blackboard jungle but the nice kids (and they are in the majority) will love it.

¹ Little Britches: Father and I Were Ranchers (1950), an autobiographical account of the early life of the American author Ralph Owen Moody (1898-1982) in the vicinity of Littleton, Colorado
² Norman Rae Taurog (1899-1981), American film director and screenwriter

The author and television columnist Charles Mercer (1917-88) used blackboard jungle in an article about the American actress Nita Talbot (Anita Sokol – born 1930), published in many newspapers on Thursday 14^th July 1955—for example in the Boston Evening American (Boston, Massachusetts):

Nita born in New York, attended public school in “blackboard jungle” area of city. She and sister spent all spare time acting out stories in park. Says:
“Act, act, act, that’s all we did. I knew then what I wanted to do and be. I don’t think the atmosphere of toughness hurt me a bit. I learned how a lot of people lived.”
Suggested good angle on her: “Beautiful blonde chalked out acting career in “blackboard jungle.”

In the following from the San Francisco Chronicle (San Francisco, California) of Thursday 17^th November 1955, blackboard jungle explicitly denotes the education system:

Education is our greatest national responsibility. How close is our system to becoming a “blackboard jungle?”
A new TV documentary on education in America poked its head over the horizon last week on KPIX—called “The Big R” (not reading, ’riting or ’rithmetic—but Responsibility). It was the first of a four-part survey—a product of group research by the TV stations in the Westinghouse Broadcasting Co. (KPIX here).