Education and Salary

Education is a service that benefits society. Educated people have better jobs, make more money and live in better neighborhoods. They are productive individuals whose higher paying jobs lead to increased tax revenue and already having much of what they need, educated people tend to commit less crime. Based on this, why are teachers' salaries so low in the United States?
The Associated Press recently reported that 1500 teachers in Detroit - who were forced to give up 5 days of pay to help balance the city budget - called in sick. This forced 53 schools in the area to close. These teachers couldn't understand why they are giving up some of their salaries to balance the budget, but at the same time principals are getting raises. The school board justified this action by comparing the salaries of principals that had recently been hired and those that had been in the system for a long period of time. Those that were just hired were earning more, and they felt that was a problem that needed to be corrected.
Is a teacher's salary equal to the benefits that education brings? Should they earn more money? Or do you think that the market for education is already in equilibrium therefore no change is needed?