I don't believe this is true. I attended a total of 7 different schools in 4 different cities in 2 different states during my primary/secondary education. Only one of these schools was notably non-diverse. I was very often exposed to children of other cultures/attitudes/backgrounds, as well as other ages. It is possible that my experience is exceptionally different, but it doesn't seem like it.
I tried to find some research on this, but unfortunately everything I find is an aggregate across all schools, rather than diversity within each school.
'Most' may have been an exaggeration, but even without, you still have social isolation in public schools, and no guarantee of it outside of them. The children in your neighborhood aren't going to be significantly different from the ones at your school.
I tried to find some research on this, but unfortunately everything I find is an aggregate across all schools, rather than diversity within each school.