Who could have guessed that growing up in a Polynesian culture is a better preparation for such a thing than going to an English boarding school..
William Golding was my father's English teacher at school (prior to publication of Lord of the Flies). According to my father, when people talked to Golding at the time, it wasn't based on real children but it definitely was based on what he believed children would be capable of.
Also Known As "[...] but in fact he made it all up."
Everything else written about the idea is speculation, from The Coral Island to The CHildren's Island to Lord of the Flies.
But Golding did observe behavior in a boarding school, and while the Tongan boys did also go to boarding school, they also were being raised in Tongan culture, and that culture, including its behavioral norms, was what helped them survive on a desert island.