My job isn't unionized, so it is fair to say that my employer pays me as little as they possibly can. Fortunately for me, there just aren't many people in the world who can do my job. The others my employer could find when they hired me either weren't as qualified as I am, or wanted too much money.
Let's set teacher's salaries the same way. Let's not argue over what they should be. Let's let schools hire and fire whoever they want, at whatever salary they want. Clearly, if they offer a salary that is too low, no qualified teacher would accept the position. The salary level will rise or fall to what is fair.
You and I both know that will result in a drop in teacher's salaries, given that there are a huge number of unemployed teachers out there, who are all qualified, and who would happily work for less than current teachers are being paid.