If a teacher can’t teach an all ability class I would ask questions of the teacher
It's an easy excuse to unload all the resposibility for teachers. In fact it's impossible to teach efficiently such class.
All kids develop at different rates so dividing them by ability at too young an age condemns the majority to inferior standards. Truly gifted kids also need to develop social maturity and separating them at too early an age can be bad for their development.
I hate that talk. In fact it's beneficial for everyone.
There is a big problem of demoralized defiant and fearless youth in gimnazja (ages: 13-16). If you segregade them at that age you will prevent many from getting demoralized. So the easy solution to increase educational standards in Poland would be introducing enrolment to gimnazja based on exam scores. Now it's a catchment area criterion.
What about boarding schools in PL?
Bursy szkolne?
If you mean private schools then there are szkoły prywatne and szkoły społeczne (społeczne as far as I know have some dotations from the gov). Both kinds are generally better than public schools if we talk about podstawówki and gimnazja, because there is no children from the bad social backgrounds. Private licea in many cases are for the drop outs, but there are exeptions too. Licea społeczne are generally good schools.