Teachers have been flexing their political muscle in Republican-dominated states where education funding was cut in recent years.
Demonstrators were inspired by a nine-day strike that last month won West Virginia's teachers their first pay raise in four years. Teachers in Arizona held another rally in late March.
Organisers were predicting a turnout of 30,000 in Oklahoma City, where aerial television images showed a large crowd, with many in red shirts, gathered outside the state capitol.
The crowd cheers during a teacher rally at the state Capitol in Oklahoma City, Monday, April 2, 2018. Source: AAP
"The message today I believe is about kids. It's about funding public education and really restoring the kind of support that all kids need in order to learn," Oklahoma state schools chief Joy Hofmeister told TV station KWTV.
Monday's walkouts shuttered some classrooms, with administrators unable to find enough substitutes to keep schools open. Other schools were already closed due to a scheduled holiday.
Some districts were facing days-long closures, as teachers vowed to continue until their demands were met.
Oklahoma City Public Schools announced its campuses would be shut through Tuesday. Hundreds of other school districts in the state also were closed, according to media reports.
Teachers by the thousands also demonstrated in the Kentucky state capital Frankfort.
The Courier-Journal newspaper said that all 120 public school districts in that state were closed although most were already on their Spring break vacation.
'Not going to take it'
Kentucky teachers were primarily protesting proposed changes to pension benefits and demanding more funding for public education.
A vocal crowd inside the state capitol rotunda in Frankfort chanted "public schools!" and sang "We're not going to take it anymore" as legislators began their work day.
Andrew Beaver, a math teacher in Louisville, Kentucky's largest city, told The New York Times that teachers were angered when the state legislature suddenly passed a bill last week to alter educators' retirement benefits.
"What I'm seeing in Louisville is teachers are a lot more politically engaged than they were in 2015 or 2016," Beaver said. "It really is a wildfire."
In Oklahoma, lawmakers recently agreed to a rare tax increase to bump teacher pay by an average of $6,100 a year. But, that was not enough to placate educators.
Some teachers said their salaries were so low that they needed second jobs -- such as working as restaurant waiters or mowing lawns -- to make ends meet.
Others said the state must do more to fund rural schools that serve low-income students.
Oklahoma teacher Jessica Lightle told CNN she was striking because of school conditions in her rural city of McAlester, where students created an online video showing the state of disrepair -- pointing to leaking ceilings and displaying textbooks that fell apart when opened.
Protesters hold signs and shout slogans as thousands of Kentucky teachers, along with student inside the Kentucky State Capitol Source: AAP
Students 'worth the investment'
Oklahoma is one of 12 states that slashed education spending following the 2008 recession and failed to restore those funds as the economy improved, according to the Center for Budget and Policy Priorities, a left-leaning Washington DC research and policy institute.
Kentucky cut per-student public education funding again in its latest fiscal plan, according to the late-2017 think tank report.
Lily Garcia, president of the National Education Association (NEA), which represents public school teachers and higher education workers -- said teachers did not trust state legislators to act without public pressure.
"This is the result of a decade of underfunding public schools," Garcia told CNN, speaking from the Oklahoma protest.
"If we don't stand up today, and say, 'our students are worth the investment,' then it's going to be too late," she said.
