Students (or their families) could choose the definition of best they wanted. Some teachers are better at adding value for less intelligent pupils, some better

Students (or their families) could choose the definition of best they wanted. Some teachers are better at adding value for less intelligent pupils, some better at pushing the more gifted, some best for the hardworking, some best for the ill-motivated ... take your pick. It can even come down to relatively inane factors like one teacher's diction being too difficult for some to follow but fine for others. Re your 2nd question: bus factor. And the same answer as the first - that you can have different definitions of best depending on the pupil.