"We're madder than hornets, and the hornets are swarming today," said Claudette Green, a retired teacher and principal.

The next Republican-voting state to join the protest movement could be Arizona, where there is an open Senate seat and where thousands of teachers gathered in Phoenix last week to demand a 20 per cent pay-rise and more funding for schools.

The growing fervour suggests that labour activism has taken on a new, grass-roots form.

"Our unions have been weakened so much that a lot of teachers don't have faith" in them, said Noah Karvelis, an elementary school music teacher in Tolleson, Arizona, outside Phoenix. Karvelis is the leader of the movement calling itself #RedforEd, after the red T-shirts protesting teachers are wearing across the country.

Karvelis said that younger teachers have been primed for activism by their anger over the election of President Donald Trump, his appointment of Betsy DeVos as education secretary and even their own students' participation in anti-gun protests after the school shooting in Parkland, Florida.

"Teachers for a long time have had a martyr mentality," Karvelis said. "This is new."

The recent strikes suggest that labour activism may not need highly-funded unions to be effective.

It follows a wave of high-impact, student-organised activism in favour of gun control, which emerged from the recent school shooting in Florida.

Unlike in strongholds, such as New York or California, teachers' unions in West Virginia, Oklahoma, Kentucky and Arizona are barred by law from compelling workers to pay dues.

Yet that has not stopped protesters from making tough demands of lawmakers.

Striking West Virginia teachers declared victory last month after winning a 5 per cent pay rise, but Oklahoma educators are holding out for more.

About 200 of Oklahoma's 500 school districts shut down on Monday as teachers walked out, defying calls from some parents and administrators for them to be grateful for what they had already received from the state.

To pay for the pay rise, politicians from both parties agreed to increase production taxes on oil and gas, the state's most prized industry, and institute new taxes on tobacco and motor fuel. It was the first new revenue bill to become law in Oklahoma in 28 years, bucking decades of tax-cut orthodoxy.

In Arizona, where the average teacher salary is $US47,000, ($61,000) teachers are agitating for more generous pay and more money for schools after watching the state slash funds to public education for years.

"We're going to continue to escalate our actions," Karvelis said. "Whether that ultimately ends in a strike? That's certainly a possibility. We just want to win."

Karvelis, 23, said teachers would not walk out of class unless they were able to win support from parents and community members across the state, including in rural areas. But he said the movement would be influential regardless of whether it shuts down schools.

"We're going to have a lot of teachers at the ballot box who I don't think would normally go in a mid-term year," he said. "If I were a legislator right now, I'd be honestly sweating bullets."

Lily Eskelsen García, president of the National Education Association, the nation's largest teachers' union, called the movement an "education spring."

"This is the civics lesson of our time," she said.

"The politicians on both sides of the aisle are rubbing the sleep out of their eyes."

New York Times, AP