Even as international studies show American students falling farther behind Asian and European students in math, science and reading scores, a group of Republican governors, mostly in Southern states, are distancing themselves from a set of education standards that most of their colleagues are embracing.

The governors find themselves under pressure from opponents of Common Core standards. Those opponents, largely made up of conservative activists, say the standards are a federal power grab aimed at dictating state education policy, which should be the domain of the states.

“There is serious public concern about the reach of the federal government into state public education policy,” Mississippi Gov. Phil Bryant (R) said in a statement Monday, when he announced an executive order that reaffirms his state’s “right and responsibility to define and implement its own public school standards and curricula.”

“Our classrooms will not become delivery vehicles for bureaucratic federal mandates. We have made tremendous progress in enacting improvements in our public education system, and we will continue pursuing what works for Mississippi children,” Bryant said.

Bryant’s order comes as Mississippi prepares to implement the standards. Forty-five states and the District of Columbia have moved toward implementation; only Minnesota, Nebraska, Virginia, Texas and Alaska have yet to act.

But even in states that have already signed on, Common Core is under a more direct threat.

The Georgia Board of Education is conducting a review of those standards, under orders from Gov. Nathan Deal (R), whose opponent in next year’s Republican primary has made Common Core an issue.

Several legislators in South Carolina are pushing to ban Common Core in next year’s legislative session; Gov. Nikki Haley (R) sounds skeptical of the national standards.

“I don’t think [Common Core is] as extreme as a lot of people paint it out to be. What I do think it is, is, you’re treating South Carolina kids like they’re California kids,” Haley said in a recent interview (A Haley spokesman, Doug Mayer, said in an e-mail: “The choice to accept Common Core was made by the previous administration and it’s now up to the legislature to reverse that and the governor supports efforts to do so.” Haley replaced Mark Sanford, another Republican.).

State legislators in Ohio, Indiana, Oklahoma, Florida, Michigan and North Carolina — all of which are controlled by Republicans — are pushing proposals to delay Common Core implementation. A conservative activist in Kentucky has challenged the standards in court, alleging that they are unconstitutional.

The second looks are coming after states accepted grants under the federal Race to the Top program even before they got a chance to see what the standards would entail, said Whitney Neal, the director of grassroots at FreedomWorks, a conservative group that opposes Common Core.

“We’ve talked to a lot of people who never read the standards,” Neal said. “They literally tell me they didn’t know what Common Core was until they already had it.”

The roots of Common Core standards grew out of Achieve, a nonprofit reform group founded in the mid-1990s aimed at crafting education standards that would lead to a workforce with the qualifications necessary for business. The initial state standards were a product of two governors — Sonny Perdue, Deal’s Republican predecessor in Georgia, and Delaware Democrat Jack Markell — working together at the National Governors Association in the late 2000s. Together, the two convinced all but five states to adopt the standards.

The Obama administration has been a cheerleader of the program — occasionally to the chagrin of those who back Common Core. Last month, Education Secretary Arne Duncan told a group of school superintendents that opposition to the new standards came from “white suburban moms who — all of a sudden — their child isn’t as brilliant as they thought they were, and their school isn’t quite as good as they thought they were.”

Duncan later apologized for his remarks, but not before opponents pounced. And the perception that Common Core is a federal program is causing some of the state push back.

“It’s now being pushed as if it’s a D.C. issue,” Haley said in a recent interview. “I still think South Carolina needs to decide this, and it’s best if we can control our environment. I’ve always felt that.”