Striking teachers on Monday picketed outside of schools and marched through Denver’s streets as car horns blared in support of the latest U.S. walkout amid a swell of educator activism that has affected at least a half-dozen states over the last year.

Just over half of the 4,725 teachers working in district-run schools called in absent for Denver’s first strike in 25 years. Some students crossed picket lines to get to class as schools remained open with administrators and substitute teachers in classrooms.

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