How can teachers demonstrate their value?

Here’s a question for parents and all other tax payers. How can teachers demonstrate their value?

It seems that people in industry are valuable to the company they work for when they bring in revenue or increase productivity. Teachers, however, don’t bring in revenue nor can they increase productivity (educate more students in this case). So, in the absence of seniority rules, which many states are trying to do away with, how do you determine the worth of a teacher?