The teacher's salary is already public information if she works for a government employer or a nonprofit that relies on transparency to encourage donations.  You are confusing employee data with business data.  Aggregate, anonymous data is available for most small business.  It's absurd to expect your doctor, or your plumber, or your used-car dealer to tell you how much they make on each or every transaction.  

This reminds me of my nosy client who considers our annual meeting an opportunity for him to drill me about my own finances.  First he asks how many clients I have, then five minutes later he asks if his fee is below or above average, then ten minutes later he asks how much rent I pay and how much I pay my staff.  Just because I know how much you make, doesn't mean you get to know how much I make.