Where did this paradigm originate that teachers were under paid? Teaching is a good and service just like engineers, doctors, plumbers, and police officers. If the field isn't paying enough for you, choose another. But it seems like we want educators exempted from economic reality. For some reason, we've adopted this idea that they're worth the same pay as doctors or even more (in some peoples' eyes). If they were worth it, the market would see to it they were paid accordingly. And if the public education system is unable to meet the expectations, maybe it's indicative that we should untether education from taxes and let the market figure it out.
Because public officials set the wages, it's not a free market. It's a public good.