As teacher resignation letters increasingly go public -- and viral -- new research indicates teachers are not leaving solely due to low pay and retirement, but also because of what they see as a broken education system.

In a trio of studies, Michigan State University education expert Alyssa Hadley Dunn and colleagues examined the relatively new phenomenon of teachers posting their resignation letters online. Their findings, which come as many teachers are signing next year's contacts, suggest educators at all grade and experience levels are frustrated and disheartened by a nationwide focus on standardized tests, scripted curriculum and punitive teacher-evaluation systems.

Teacher turnover costs more than $2.2 billion in the U.S. each year and has been shown to decrease student achievement in the form of reading and math test scores.

"The reasons teachers are leaving the profession has little to do with the reasons most frequently touted by education reformers, such as pay or student behavior," said Dunn, assistant professor of teacher education. "Rather, teachers are leaving largely because oppressive policies and practices are affecting their working conditions and beliefs about themselves and education."