Most members of Mensa have some kind of education horror story to tell, which might be the reason that homeschooling is popular in the group.
Mensa is a high-IQ society. Membership is based on scoring at the 98th percentile or above on any of dozens of approved, standardized tests if general intelligence. Generally, Mensans can complete high school with good to excellent grades without ever being challenged or forced to study. Their natural curiosity is usually enough to carry them through the 12th grade, but they often hit a wall when they enter college.
That happened to me at MIT, and to several of my friends at various other colleges. But my parents were fervent believers in public education, and would NEVER let me skip any grades or attend a private school, although from the 7th grade on they certainly could afford it. I finally got my first degree decades later.