Granted, my experience is from podunk USA, where the school population was very low-- but:
My big problem was that the school had certain... materialistic needs.. that were predicated on meeting federal competency requirements, coupled with students that were academically lazy, due to a complete lack of consequences on their part.
For instance, the school could fail to meet mandated grade scores if the students were graded accurately. This placed a strong bias pressure on teachers and administrators to concoct testing scenarios that would incorrectly pass students that totally did not understand the subject matter (tm) as those that Totally Did Understand(tm).
To maintain the charade, those students that Totally Did Understand, and did so legitimately, were often saddled with a large number of other students who did not, and given "Group activitites", so that the contribution of those "Good ones" would bring up the aggregate scores of those that did not, allowing them to pass, and for the school to stay funded.
Nothing sucks the life out of school like being burdened in this way-- Either for teachers, or for the students.
Instead, I feel that identifying students that would benefit from supplementary education is not only a good idea, but is essential. It profits nobody, NOBODY, to find clever ways to pass these students through the system, just so the school can stay funded. Likewise, it does not profit the lower-achievers to put the brainiac with them-- because understanding is not transmissible. (It has to be built on, and the empirical fact that the brainiac picked up the material quickly while the lower-achiever did not, indicates that they have different basic understandings from which to grow to the desired level. What makes sense to the brainiac is confounding to the others. It is not good to pair them up.)
Underlying this whole problem is the philosophical notion that all students are essentially equal. They aren't. Life experiences, interests, personality quirks-- everything-- all conspire to determine if a specific, individual student is going to take to a specific topic with ease, or with difficulty. Treating all students like tabula rasa is bullshit.
Identifying those in need of more aggressive search for underlying understanding nuggets to build from, so that they can get that attention, is in everyone's interest.
But for some reason, politicians and parents alike consider this to be a bad thing, and I can't for the life of me understand why.