“In the history of education, the most striking phenomenon is that schools of learning, which at one epoch are alive with a ferment of genius, in a succeeding generation exhibit merely pedantry and routine.”

Einstein did write this quote in "On Education" from 1936, which appeared in Out of My Later Years, but it was not his own original quip, he attributed it to an unnamed "wit".
Very popular in French: "La culture est ce qui reste lorsque l’on a tout oublié" (Culture is that which remains, if one has forgotten everything). Attributed in French to Édouard Herriot (1872-1957) and, in English, sometimes to Ortega y Gasset. Another French variant is "la culture est ce qui reste lorsqu'on a oublié toutes les choses apprises" (Culture is that which remains if one has forgotten everything one has learned), which appears in the 1912 book Propos Critiques by Georges Duhamel, p. 14 http://books.google.com/books?id=Xpk_AAAAIAAJ&q=%22la+culture+est+ce+qui+reste+lorsqu%27on+a+oubli%C3%A9+toutes+les+choses+apprises%22#search_anchor. And another English variant is "Culture is that which remains with a man when he has forgotten all he has learned" which appears in The Living Age: Volume 335 from 1929, p. 159 http://books.google.com/books?id=tHFRAAAAYAAJ&q=%22Culture+is+that+which+remains+with+a+man+when+he+has+forgotten+all+he+has+learned%22#search_anchor, where it is attributed to "Edouard Herriot, French Minister of Education". Another English variant is "Education is that which remains behind when all we have learned at school is forgotten", which appears in The Education Outlook, vol. 60 p. 532 http://books.google.com/books?id=dNcgAQAAMAAJ&pg=RA1-PA532#v=onepage&q=%22education%20is%20that%20which%20remains%22&f=false (from an issue dated 2 December 1907), where it is attributed to Ralph Waldo Emerson.
The saying is found in an 1891 article by Swedish writer Ellen Key https://en.wikipedia.org/wiki/Ellen_Key, "Själamorden i skolorna", which was published in the journal "Verdandi", no. 2, pages 86-98 (the saying is on p. 97). The same article was republished later as a chapter in her 1900 book "Barnets Århundrade". Here is the quote in Swedish ( p. 160 https://archive.org/stream/barnetsrhundrade02ellenkey#page/n167/mode/2up): Men bildning är lyckligtvis icke blott kunskap om fakta, utan enligt en ypperlig paradox: »det, som är kvar, sedan vi glömt allt, vad vi lärt». Here it is from the 1909 English translation of the book ( p. 231 https://archive.org/stream/centurychild00frangoog#page/n246/mode/2up): "But education happily is not simply the knowledge of facts, it is, as an admirable paradox has put it, what is left over after we have forgotten all we have learnt." From the way Ellen Key puts it, she doesn’t take credit for the saying, but rather refers to it as an already known “paradox” that she explicitly puts between quotation marks.
Misattributed

The Wonder that was India (1954).
Context: The age in which true history appeared in India was one of great intellectual and spiritual ferment. Mystics and sophists of all kinds roamed through the Ganga Valley, all advocating some form of mental discipline and asceticism as a means to salvation; but the age of the Buddha, when many of the best minds were abandoning their homes and professions for a life of asceticism, was also a time of advance in commerce and politics. It produced not only philosophers and ascetics, but also merchant princes and men of action.

"Reversing Established Orders", p. 402
Leonardo's Mountain of Clams and the Diet of Worms (1998)

Studies in Greek and Roman History, Alexander the Great and the Loneliness of Power, 1964 p. 204
Context: After fighting, scheming and murdering in pursuit of the secure tenure of absolute power, he found himself at last on a lonely pinnacle over an abyss, with no use for his power and security unattainable. His genius was such that he ended an epoch and began another - but one of unceasing war and misery, from which exhaustion produced an approach to order after two generations and peace at last under the Roman Empire. He himself never found peace. One is tempted to see him, in medieval terms, as the man who sold his soul to the Devil for power: the Devil kept his part of the bargain but ultimately claimed his own. But to the historian, prosaically such allegory, we must put it differently: to him, when he has done all the work - work that must be done, and done carefully - of analysing the play of faction and the system of government, Alexander illustrates with startling clarity the ultimate loneliness of supreme power.

Growing Without Schooling magazine, no. 40 (1984).

Chap.II: The Rise Of The Historic Level
The Revolt of the Masses (1929)

Source: What I Talk About When I Talk About Running

Interview by David Sheff in Rolling Stone Twentieth Anniversary Issue (1987)
Context: We are dealing with the best-educated generation in history. They are a hundred times better educated than their grandparents, and ten times more sophisticated. There has never been such an open-minded group. The problem is that no one is giving them anything fresh. They've got a brain dressed up with nowhere to go.