In the fifties and sixties about a quarter of school kids passed the 11+ and went to grammar school. So what was the IQ threshold?
Based on a normal distribution with mean IQ 100 and standard deviation 15, this equates to an IQ of 110 to get into grammar school. To get into the top 20% of the grammar school you would need an IQ of 125.
With 40% of eighteen year olds going to university, that IQ threshold is just 104.