Suppose your child is very gifted academically. You would want your child to attend school where there were other bright kids. Should there only be bright kids in the school? Sounds tense and ultra competitive.
If your child struggled to read, should he/she only go to schools with children who have similar difficulties. Perhaps your child scores in the middle academically, with whom should this child attend school?
In a world of school choice, schools tend to get stratified by test scores and school grades. An average kid can choose a school with average kids. Is this what we want for our children?