When I was a kid, public schools were awesome because kids were tracked. I was tested and then put into the college track (even though I was low income, so it surprised the teachers.) Other kids ended up in the vocational training track or the ditch digging track. It was a great system. The worthless loser kids were all put together and the kids who wanted to make something of their lives were put in different classes. Everybody won (except for the worthless loser kids, but they weren't going to win anyway and at least we didn't get dragged down by them.)
I think the problem with this (as I've read) is that by mixing students together, lower/ middle achieving students actually achieve more.
Hence, you have mainstreaming of disabled students and mixing of levels of students.
Of course, the bad side of that is that the advanced students/ GATE students are harmed by this. On one hand, it's good for them that they get to help others. I know I tutored a lot in HS (unofficial tutoring during math class). On the other...they get further behind.
Our school maintstreams and blends. It's okay if they have regular "pull outs" for reading, math, etc - which they had in the lower grades (son is going into 5th). From talking to his 4th grade teacher and a 5th grade teacher next year ... I mean, in 4th grade, there were kids at the 1st grade level in math, and 2nd grade level in reading. (My son was ~7th grade at the start of the year.)
The 5th grade teacher, in addition to noting the EXTREME differences in abilities among the students, also noted that about half the kids are "done" with school by then. Whether it be lack of support at home (either parents working too hard/ 2 jobs or just not invested in education), the lack of personal responsibility, or whatever - they don't want to be there anymore.
So of course, what the school WANTS to do is to continue that. Many many parents have been requesting "tracks" or "clusters". The teachers obv don't want to do it - who wants to be the 5th grade teacher with the kids performing at a 3rd grade level?? But this year, we had an unprecedented 12 students make GATE in second grade (~ 20% of the grade). We have a hard time keeping these students, as the district has a "GATE magnet" (gate only students) program at a different school, which is unfortunately only a mile down the road. (So, no geographical barrier.)
How many students will we lose? I hope not many. But we are trying to convince the new principal and the staff that what they want to do is do a cluster/ grouping. Face it - in our school, it's the higher SES families that donate all the money and most of the time. 20% of the families. It's a struggle to raise even $60k a year (the school 1 mile away? >$500,000 a year). If we don't change things, we continue to lose families, and the money that goes with them.