Yes, the "system" ensures that talented teachers become disillusioned fast and move on. Sometimes, head hunted to private schools. This teacher would not display this enthusiasm unless his students are also somewhat "selected". Sometimes, subjects, at senior level in the public arena are cancelled when a class list contains the names of students well known to teachers at the school and who would struggle hugely with the subject on offer. It happened to me. Not much fun talking to the walls, really. Nor looking at the blank stares of vacant eyes 40 minutes at a time, 5 sessions a week. Especially, if that describes the majority of the students. Genetics plays a huge part in what can and cannot be sensibly offered as subject matter to senior students.
