It's on a scale from "Nothingburger, you 'gifted' kids just want to feel special" to "MKULTRA PROJECT MONARCH PROGRAMMING".
I personally am in the category of those who think there was something to it. It didn't have to be anything as extreme as like, X-Men, but there were active projects in things like education at the time, and with the rise of standardized testing, I do see the government (and/or private entities) being willing to put money into school type and curriculum experiments. If nothing else, it would explain the suspiciously-high number of these programs that came out of seemingly nowhere and how they were rolled out so quickly.
Think about it. It's almost impossible to change school curriculums these days (both politically and financially), so how were these schools/districts able to do it so quickly during this time period? Where did the money and the curriculum come from?