They are largely underpaid, but in the past few years, people on the right have decided to target public employees in general and teachers in specific (for some reason) as being The Problem with America.

Teachers' modest salaries, have, for a long time, been countered with good benefits in other areas: pensions, health care, vacation time, job security. Most working Americans in the private sector have none of those things anymore, so the right points at the teachers and says "see, this is the problem right here, these public sector health plans and pensions!"