School teachers in the US today are being asked to do more with less, and many of them have had enough. The groundswell of unrest that began with a teachers' strike in West Virginia has spread to several other states. Frustrated teachers are staging walkouts , holding "walk-ins" and lining up to run for office

How did things get this bad? Here are seven charts that help explain the problem.

Teaching is among the most respected professions in the US ...

... and yet teachers' wages are lower than those of other college graduates

You can finish school, take pretty much any white-collar job -- accountant, human resources manager, whatever -- and earn more than a teacher.

For years, their salaries have remained flat ...

Salaries for US secondary school teachers have largely remained the same over the past two decades, according to the National Center for Education Statistics.

... and their pay is falling behind that of other workers

In 2010, teachers earned 12% less than workers in similar professions. By 2015, that gap had grown to 17%.

In many states, funding for schools is plummeting

Some of the states that have cut money for schools the most in recent years -- Arizona, Oklahoma and Kentucky -- are also the ones currently grappling with teacher strikes and walkouts.

No wonder more teachers are leaving the profession ...

Two-thirds of US teachers quit before retirement, according to the Learning Policy Institute . Their main reasons? Low salaries, dissatisfaction with testing and accountability pressures, lack of opportunities for advancement and dissatisfaction with working conditions.

... and the supply of aspiring teachers to replace them is drying up