The other thing you touch on is student doing some preparation. Most don't, I never did, and now I am trying to get my son to do some before his lessons. It saves a lot of money.
Thats because they are not supervised and directed to. Instruction doesn't just stop 'on blocks'. Instruction should include directed and supervised learning away from the school as well. All of my students receive a handout for each exercise which directs learning & includes a progress test. Obviously I cannot make them do it but when I explain to them after two failures to do the work that it would be better if they discontinue training it always seems to have the desired effect! Sloppy standards produce sloppy students!