Over the past 25 years, teacher salaries have fallen drastically behind. It’s a phenomenon called the “teacher pay gap,” and two years ago, it reached an all-time high.

Public school teachers earned 19 percent less than workers with similar levels of education and experience in 2017, according to an analysis of the most recent data by the Economic Policy Institute. That’s up from just a 3 percent gap in the mid-90s.

All across the country last year, public school teachers walked off the job to demand better resources and higher pay. The states that have some of the largest pay gaps in the country — like Arizona, Oklahoma or Colorado — saw the biggest surges in teacher activism.

"The issue we're really talking about here is that we have a constant defunding of teacher pay as we have an erosion of public investment in the public school systems more generally,” said economist Sylvia Allegretto, one of the authors on the Economic Policy Institute’s Study. She explained that as public school budgets were cut during the Great Recession, they weren't reinstituted during the economic recovery that followed.

“So that really plays into a labor market that has been broken for a long time, for a lot of people,” Allegretto added.

The movement shows no signs of slowing down in 2019 either. After a successful six-day teacher strike in Los Angeles earlier this month, teachers in Denver, Virginia and elsewhere in California, are either protesting or getting ready to strike.

This segment originally aired January 29, 2019, on VICE News Tonight on HBO.