While it's hard to prove a negative, it is extremely unlikely that Nabokov based this passage on any kind of real-world or second-hand knowledge.
As the essay The Long 1950s in the collection Vladimir Nabokov in Context makes clear, sexuality was largely a taboo subject in 1950s America. Media around sexuality largely reinforced the image of the nuclear family, adultery was still considered a crime and women were portrayed as mothers and caregivers rather than sexually active beings. This was, to some extent, a backlash against the relative sexual and economic freedom women had enjoyed during World War 2.
This would have made it difficult for Nabokov to get the information needed to "prove" his assumption about the sexuality of schoolgirls was realistic. He had no daughters, only one son, Dmitri. Given the prevailing culture of secrecy around sexuality, and the fact children tend not to discuss sexual matters with their parents even today, it's therefore unlikely that he could have learned about what really went on in the social lives of teenage girls in the 50s.
Lolita makes several digs at The Kinsey Reports which were the first serious attempts to make a study of the sexuality of Americans. These would have been the only literature available at the time from which he might have been able to get more anonymised or statistical data to prove the realism of his scene. One of their more striking conclusions, interestingly, was to blow open the reality of the sexual abuse of children, a subject that had previously been considered totally taboo.
However, despite his making reference to them in the book there is no evidence he actually read them, and a degree of circumstantial evidence that he did not. The references in Lolita are mocking in nature, suggesting he didn't think much of them. He had a longstanding dislike of psychiatry and psychology and lumped Freud with a group of intellectuals he described as "fake thinkers and puffed-up poets". There are many other examples from interviews and writings, including:
I think he's [Freud] crude, I think he's medieval, and I don't want an elderly gentleman from Vienna with an umbrella inflicting his dreams upon me. I don't have the dreams that he discusses in his books. I don't see umbrellas in my dreams. Or balloons.
I think that the creative artist is an exile in his study, in his bedroom, in the circle of his lamplight. He's quite alone there; he's the lone wolf. As soon as he's together with somebody else he shares his secret, he shares his mystery, he shares his God with somebody else.
This is further suggestive that Nabokov would perhaps not have delved into such a secretive and sensitive topic as sexuality with others and, even if he had, he would certainly not have admitted to it. Given his aversion to the study of psychosexual topics, it would seem rather unlikely that he would have bothered reading either of the Kinsey reports.
So there's simply no route through which we can imagine Nabokov receiving the information he would need to have had Lolita's sexual experiences inspired by real life. Indeed, given the dearth of contemporary sources on the matter, it's likely we'll never get a clear insight into the reality of teenage sex in the 50s.
References:
Nabokov's Memory War against Freud, TECKYOUNG KWON, American Imago, Vol. 68, No. 1, From Freud to Literature (Spring 2011), pp. 67-91
Vladimir Nabokov in Context, Ed. David M. Bethea & Siggy Frank, Cambridge University Press, 2018
