Well, everyone has one's limits in everything - be it the weight one can lift, or the speed at which one can run, or one's ability to absorb and profit from education. Thus the only proper and necessary form of educational segregation is by ability - into separate and by design unequal educational streams. From the retardees to the gifted and maybe even "super-gifted" - at one extreme they will struggle with the multiplication table within 6x6, while at another they will be doing [in their heads] multidimensional geometry as visualization exercises. To each one's own.
That's how it used to be, educational segregation. When I was a lass my public school had separate classes for math and reading, based solely on skill level. In math we went to different classrooms; in reading our regular class was split into colored "zones," again based on skill level. In different areas of the room each zone would pursue goals based on that group's abilities: remedial phonics, reading primers, grade-level textbook stories or SRA cards, advanced texts or trips to the library for extra materials. The advanced group even did weekly sessions with younger grades teaching reading skills.
Sure, there were mild embarassments being part of a particularly low-skill zone, and a bit of pride in being in the top zone, but the beauty of the scheme is that the kids could apply themselves and move into other zones as their skills progressed. Hard work = move up. What a concept, huh?
A decade later there was so much emphasis placed on "feelings" and social skills that pursuit of the three Rs was relegated to a secondary goal. Dumbing down, not building up, became the norm. Sensitivity became more important than ability. Answers became subjective, not definitive. It's no wonder society is in its current state.
(For the record, I homeschool my own kids. I wouldn't send them to public school for a million dollars.)