Teacher Salaries

How do you suggest they differentiate between bad classes and bad teachers?

You might disagree, Kevin, but Im a believer that a good teacher is able to make a bad class do well. Good students will get the grades regardless of who's teaching them. You don't need a good teacher to get them to do well. A good teacher is measured by what they can do with the poorer students in the class and make them excel (to some degree).