These charts show why America's teachers are fired up and can't take any more

Updated 6:41 PM ET, Thu May 3, 2018

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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.