The army in red began to assemble before sunrise — at busy street corners, on overpasses and outside hundreds of schools across the city — wielding boxes of doughnuts, picket signs and (for now, at least) a fierce determination.

They banged out an urgent drum beat. They yelled. They wore wacky costumes — a T. rex, a giant hot dog. Whatever it took for Chicago Public Schools teachers and support staff to draw attention to the cause on Day One of their strike.

“I’m willing to strike for as long as it takes to get a fair contract,” Scott Plencner, a history teacher at Taft High School on the Northwest Side, said Thursday morning. “We’re teachers. Sometimes we’ve got to teach the mayor.”

Within yelling distance, about a dozen of Plencner’s colleagues spread out along an overpass above the Kennedy Expressway, waving signs at the roaring traffic below and urging the big rig drivers to honk their horns, which they mostly did.

About four miles to the south, at Reinberg Elementary School, Pat Grabowski, a veteran special education teacher, was predicting a short strike if Mayor Lori Lightfoot hopes for a second term, she said.

“If she doesn’t nip this in the bud, it will affect her reelection,” Grabowksi said. “I think she’ll have a deal by Monday.”

A quick end to the strike was the wish of lots of parents, many scrambling to find a safe place for their school-age kids.

Lane Tech High School AP psychology teacher David Doll brought his daughter, Penny, 4, to the picket line.

“Explaining the strike to her was a little difficult,” Doll said. “I told her, ‘Everyone has piggy banks. We need more money for our piggy bank and our school needs more money for its piggy bank. And she was cute — she went and got her piggy bank and brought it to me.”

Jude Greiner, 18, a Lane Tech senior, walked the picket line — handing out coffee, water, doughnuts, granola bars and crackers.

“My teachers are striking so everyone has equal resources,” Greiner said. “I hope the city understands. My school is very lucky, but every school needs a nurse and proper resources for their children.”

Across the city, parents and countless others showed their support for the striking teachers — even if it was just honking a car horn on the way to work.

At Kelly High School in the Brighton Park neighborhood, band students showed up playing their instruments. The school’s Youth Alive Bible Club visited too, bringing tamales.

“Tamales are great strike food,” said senior Patricia Vazquez. “They’re warm and you can eat them standing up.”

Contributing: Mitch Dudek, Carlos Ballesteros