| Many of the greatest geniuses, such as Newton, Maxwell, and Einstein—were all viewed as ‘dunces’ in childhood were labeled and or considered, by teachers, parent, or themselves, as dunces in childhood.  As Thomas Edison stated himself, on his experience at Port Huron school: |
“I used never to be able to get along at school. I don’t know what it was, but I was always at the foot of the class. I used to feel that the teachers never sympathized with me and that my father thought I was stupid, and at last I almost decided that I really must be a dunce.”It seems to be an "inverse genius relationship phenomenon" of sorts.
Those who many consider the three greatest scientific minds of all time, namely: Newton, Maxwell, and Einstein, to cite a commonly discussed example, were all viewed as ‘dunces’ in childhood, but in historical retrospect resulted to be great geniuses. 
Shown below, ordered by date of synthesis (birth), are some of the more famous examples amid the top 1000 geniuses to have been labeled or mis-labeled as non-geniuses in youth:
Polish astronomer and general polymath Nicolaus Copernicus (1473-1543), whose On the Revolution of the Heavenly Spheres initiated the scientific revolution, was, supposedly, assigned by Lewis Terman, the inventor of the IQ scale, with an IQ of 100 to 110, i.e. just above average in intellect?
“The American hereditarian Lewis Terman, the man most responsible for instituting IQ tests in America, retrospectively calculated Gallon's IQ at above 200, but accorded only 135 to Darwin and a mere 100-110 to Copernicus (see pp. 213-218 on this ludicrous incident in the history of mental testing). Darwin, who approached hereditarian arguments with strong suspicion, wrote after reading Hereditary Genius: ‘You have made a convert of an opponent in one sense, for I have always maintained that, excepting fools, men did not differ much in intellect, only in zeal and hard work’ (in Galton, 1909, p. 290). Galton responded: ‘The rejoinder that might be made to his remark about hard work, is that character, including the aptitude for work, is heritable like every other faculty’.”— Stephen Gould (2006), The Mismeasure of Man 
English physicist Isaac Newton (1642-1727), supposedly, did poorly in grade school. 
Tolstoy | Flunked college
Russian existentialism writer Leo Tolstoy (1829-1910), noted human particle theorists, presently ranked as a top 25 greatest literary author ever, flunked out of college. 
American inventor Thomas Edison (1847-1931), as a boy, was told by his teachers that he was too stupid to learn anything. 
Poincare | IQ:35 [?]
French physicist Henri Poincare (1854-1912) famously so poorly on the Binet IQ that he was judged an imbecile (IQ=35); although we now rank him at IQ=195.
British politician and Nobel laureate Winston Churchill (1874-1965) failed sixth grade. 
Albert Einstein (1879-1955), supposedly, was three or four years old before he could speak and seven before he could read—he was born with a misshapen head: as a result, his parents feared he was mentally retarded; he so withdrawn or "set outside the group" that one governess nicknamed him ‘Father Bore’. Einstein latter attempted to skip high school by taking an entrance exam to the Swiss Polytechnic, a top technical university, but famously failed the art portion.
American chemist Linus Pauling (1901-1994) who we now rank, in retrospect, at IQ=190, notably, failed to take some required American history courses and did not qualify for his high school diploma. The school awarded him the diploma 45-years later only after he had won two Nobel Prizes.
American illustrator turned movie mogul Walt Disney (1901-1966) (IQB=123) was fired as a newspaper editor because, supposedly, he had “no good ideas”. 
American electronics physicist William Shockley (1910-1989), known to some as the “creator of the electronic age”, whose semi-conductor work and short-lived semiconductor laboratory (Ѻ) became the vicarious epicenter of Silicon Valley, as a youth, was one of the hundreds of children in Palo Alto, San Francisco, and Los Angeles, that psychologist Lewis Terman, in 1916, began testing for entrance into his gifted program, a score of 135 or above—Terman’s delineation of genius level cutoff—needed for entrance. William took Terman’s IQ test twice, scoring 129 at age 8 and 125 at age nine, thereby failing to make the cut for entrance into Terman’s genius program. William, in later years, would often joke on the irony about how he could not qualify for Terman’s 1921 gifted study, yet could still win a Nobel Prize in physics. 
In c.1921, Luis Alvarez (1911-1988), age 10, failed to gain entrance to the Terman gifted study, meaning his IQ was tested at 134 or below, but went on to win a Noble Prize in physics for his work in particle physics, and to become famous for his "asteroid impact extinction theory".
Feynman | IQ:123?
American physicist Richard Feynman (1918-1988), one of the greatest geniuses of the last century, as a kid, according to his sister Joan, was tested with an IQ score of 123, i.e. below genius level—or as reported by biographer James Gleick, in high school his IQ was determined to be 125. 
In 1992, Ayaan Ali sought political asylum in the Netherlands and in 1997 citizenship, during which time she took an IQ test; retrospect comments on her score are as follows:
“I point out that Ayaan Hirsi Ali was given an IQ test in the Netherlands and did very poorly. Yet, it’s hard to imagine someone brighter.”— Jason Richwine (2008), panel, at the American Enterprise Group, discussing (Ѻ) new book by Mark Krikorian, director of Center for Immigration Studies
In 2017, Ali was RGM ranked as 521st greatest mind of all time out of 1,500+ historical names.
American electrochemical engineer Libb Thims, cited by popular vote (2012) as having the "highest IQ ever", among other frequent genius, polymath, oracle (or prophet), etc., citations and or queries (see: Thims genius ranking), main curator of the only subject common to IQ: 225+ cited scholars, namely human chemical thermodynamics, scholars notably "flunked" second grade (made to retake the entire year), owing to teacher reports that he was "bored in class", and thereafter stayed at or below average level, until the point of high school graduation (age 19). At one point, sometime between age 12 and 17, the entire school was made to take the state mandated ACT test, during which time Thims started answering some of the questions, but after a certain point, just started filling in all the columns with the same answer, e.g. a, a, a, a, b, b, b, b, c, c, c, c, etc., just be done with the test, and generally per reason that he really didn't care, resultantly scoring 19, which correlates to an IQ of 97. 
See also● Feynman (2014) – Hmolpedia thread.
● Paper IQ
1. Pickover, Clifford. (1998). Strange Brains and Genius: the Secret Lives of Eccentric Scientist and Madmen. New York: Quill.
2. Shurkin, Joel N. (2006). Broken Genius: the Rise and Fall of William Shockley, Creator of the Electronic Age (pg. 13). Palgrave MacMillan.
3. Characteristics and Behaviors of the Gifted (flunk section) – RI.net.
4. Gleick, James. (1992). Genius: The Life and Science of Richard Feynman (IQ:125). Pantheon Books.
5. Gelb, Michael and Caldicott, Sarah M. (2007). Innovate Like Edison: the Success System of America’s Greatest Inventor (pg. 59). Penguin.
6. Gould, Stephen. (2006). The Mismeasure of Man (pg. #). Publisher.
7. SAT, ACT, and IQ – HomeOFBob.com.