“It affects their morale in the classroom,” she said. “The last thing we want is our teachers worried about how they are going to pay their bills.”

The average salary for a public school teacher nationwide in the 2009-10 school year was $55,350, a figure that has remained relatively flat, after being adjusted for inflation, over the last two decades. Starting teacher salaries can be significantly lower; compared to college graduates in other professions, they earn more than $10,000 less when beginning their careers.

“I think people have felt the need to supplement their teaching salaries in order to have a middle class lifestyle,” said Lawrence Mishel, president of the Economic Policy Institute, which published a study this year concluding the average weekly pay of teachers in 2010 was about 12 percent below that of workers with similar education and experience.

The Organization for Economic Cooperation and Development, which collects data on student performance across the globe, advised the United States earlier this year to work at elevating the teaching profession in order to improve student performance. The recommendations included measures like raising the bar for who is selected to become a teacher, providing better training and better pay. In many nations where students outperform the U.S. in reading, math and science, including Japan and South Korea, teachers earn more than they do in the United States.

“International comparisons show that in the countries with the highest performance, teachers are typically paid better relative to others, education credentials are valued more, and a higher share of educational spending is devoted to instructional services than is the case in the United States,” the OECD report concluded.

While moonlighting isn’t unique to teachers, they do tend to have second or third jobs at a higher rate than other professionals. One researcher estimates their moonlighting rates may be four times higher than those of other full-time, college educated salaried workers.

Eleanor Blair Hilty, an education professor at Western Carolina University, said most teachers make around $5,000 through outside work. Yet when asked if they would quit if given a raise in the equivalent amount, most said no. Her conclusion: teachers are getting something more from their second job other than an extra paycheck.

“A lot of it has to do with what I think is wrong with the teaching profession,” Hilty said, noting that teachers have little autonomy and control over what and how they teach. “They found their moonlighting jobs to be satisfying.”