Of American-English origin, the colloquial phrase the school of (the) hard knocks denotes the experience of a life of hardship regarded as a means of instruction.
—Synonym: the university of life.

The phrase the school of (the) hard knocks occurs, for example, in Expelled students need education, too, an editorial published in the Albuquerque Journal (Albuquerque, New Mexico, USA) of Wednesday 14^th June 2023—APS stands for Albuquerque Public Schools:

With so many school shootings in recent years, it’s not surprising parents are concerned about the safety of their children on campus.
[…]
In response to safety concerns, APS officials are considering a procedural directive that would bar the school district from enrolling any student who’s been expelled from any public or private school in the past 12 months in New Mexico or elsewhere, and students who committed expellable offenses in the past 12 months but weren’t actually expelled.
[…]
But what happens to that student if no other school will enroll him or her? […]
APS appears to have no plan other than to turn them away, beginning this fall.
We must remember these are children with their adult lives ahead of them. They’ll need a solid education to succeed, not 12 months in the school of hard knocks.

The earliest occurrences of the phrase the school of (the) hard knocks that I have found are as follows, in chronological order:

1-: From The Men Who Advertise; An Account of Successful Advertisers, Together with Hints on the Method of Advertising (New York: George P. Rowell & Co., 1870), by the U.S. advertising agent and publisher George P. Rowell (1838-1908)—the following is about “John W. Pittock, the editor of the Sunday Leader in Pittsburgh”:

His misfortunes were largely owing to the inexperience of youth. Trained, however, in the school of hard knocks, he now had learned the theory of success, and from that time on has had it.

2-: From the Vermont Christian Messenger (Montpelier, Vermont, USA) of Thursday 15^th November 1877:

The School of Hard Knocks.—A great deal of useless sympathy is in this day expended upon those who start out in life without social or monetary help. Those are most to be congratulated who have at the beginning a rough tussle with circumstances. John Ruskin ¹ sets it down as one of his calamities that in early life he had “nothing to endure.” A petted and dandled child makes a weak and insipid man. You say that Ruskin just quoted disproves the theory. No! he is showing in dejected, splenetic, and irritable old age the need of the early cudgeling of adversity. A little experience of the hardship of life would have helped to make him grateful and happy now. No brawn of character without compulsory exertion. The men who sit strong in their social and financial elevation are those who did their climbing. Misfortune is a rough nurse, but she raises giants. Let our young people, instead of succumbing to the influences that would keep them back and down, take them as parallel bars, dumb bells, and weights of gymnasium, by which they can get muscle for the strife. Consent not to beg your way to fortune, but achieve it. God is always on the side of the man who does his best. God helps the man who tries to overcome difficulties.—Exchange.

¹ This refers to the British author and artist John Ruskin (1819-1900).

3-: From the Buffalo Morning Express (Buffalo, New York, USA) of Saturday 26^th April 1879:

A HERESY NEEDING MORE KILLING.

Most mature men remember that fourteen years ago there terminated in this country a great and destructive civil war ². It was fought to settle one question—and one only: Whether the individual States or the Central Government had supreme authority. […]
[…]
True, the war gave the Federal Government no new right. But the war confirmed and established that right of the Government which the followers of Calhoun ³ had always denied to it: the right of sovereignty; the right to be a nation, complete and supreme in its functions, in no wise dependent on the assent or dissent of the jarring and discordant States which are its members, having their own proper but unmistakably subordinate powers.
This is the lesson of the war, which the Confederate statesmen now rearing their heads again at Washington should have learned in that school of hard knocks which was established especially for their benefit. If they force the nation to give them a second lesson on the same subject more pains will doubtless be taken to make them heed and remember the teaching.

² This refers to the American Civil War (1861-65).
³ A Senator from South Carolina, John Caldwell Calhoun (1782-1850) was the Senate’s most prominent states’ rights advocate, and his doctrine of nullification professed that individual states had a right to reject federal policies that they deemed unconstitutional.

4 & 5-: From transcripts of lectures that the U.S. preacher Thomas De Witt Talmage (1832-1902) delivered:

4-: At Des Moines, Iowa, on Monday 10^th March 1884—transcript published in The Evening Gazette (Cedar Rapids, Iowa, USA) of Wednesday 12^th March 1884:

We are graduates from the school—the school of hard knocks. Some sit down and repine, others rise up and conquer. Happy he who can find some comfort in any adversity.

5-: At Montreal, Quebec, on Wednesday 4^th June 1884—transcript published in The Montreal Daily Star (Montreal, Quebec, Canada) of Thursday 5^th June 1884:

There were many colleges in the world but there was only one that everybody went through—the school of hard knocks, in which misfortune and care were the professors.

6-: From A Lecture to the Boys and Their Parents, published in the Press and Horticulturist (Riverside, California, USA) of Saturday 29^th November 1884:

Perhaps some of the more ambitious [boys] wish to continue their studies through the high school and the Normal, and elbow their way at last with the great throng of applicants for positions as teachers in our public schools. Some may choose a college education; and after spending years in the impractical work of conjugating Latin and Greek verbs, find themselves at the age of twenty-four or five mere children in the ways of the world. We would not decry higher education. But experience has shown us so many, many men cut off to the same length of mediocrity by its Procrustean bed—so many schooled to dependence on their fathers’ bounty through all the years of their youth, graduating at last mere educated pigmies, unable to force their way to the front in the practical callings of life—thrust aside by the boys from the school of hard knocks and self-maintenance, that we sometimes question the advisability of so much higher education.