U.S. Education Secretary John King on Tuesday urged Colorado and 21 other states to stop allowing corporal punishment in schools, a practice he called “harmful, ineffective, and often disproportionately applied to students of color and students with disabilities.”

The nation’s education chief instead advocated the use of disciplinary measures that create a positive school climate and promote nonviolent techniques for conflict resolution.

King outlined his concerns in a letter to the governors and education chiefs of states that allow disciplinary techniques such as paddling or spanking.

“… The very acts of corporal punishment that are permissible when applied to children in schools under some state laws would be prohibited as criminal assault or battery when applied to adults in the community in those very same states,” King wrote in his letter.

Corporal punishment is defined as intentionally inflicting pain to punish a child or in an attempt to change behavior. The U.S. Supreme Court ruled in 1977 that the practice does not violate constitutional rights.

Fifteen states, most of which are in the South, have policies allowing corporal punishment in public schools. Colorado is one of seven states that lacks a policy banning corporal punishment.

Colorado law requires districts to have clear policies on the use of physical force in dealing with disruptive students and states that discipline codes can’t conflict with child abuse rules.

If the state did ban corporal punishment, it would likely have to come in the form of legislation. Jeremy Meyer, spokesman for the Colorado Department of Education, said if the State Board of Education created such a policy, it would probably be seen as overreach and be flagged by the state’s Office of Legislative Legal Services.

The use of corporal punishment is rare in Colorado, according to the most recent federal data. In 2011-12, it was used on 485 students in the state, or about .05 percent of all Colorado students.

In contrast, the practice was used on tens of thousands of students in states such as Mississippi, Texas and Alabama, according to the U.S. Department of Education’s Office of Civil Rights.

Data collected by that office shows a racial gap in how corporal punishment is dispensed. Nationally, black boys are 1.8 times as likely as white boys, and black girls are 2.9 times as likely as white girls, to receive the disciplinary measure.

And in nearly all of the states where the practice is permitted, students with disabilities were subjected to corporal punishment at higher rates than students without disabilities. Boys are also disproportionately impacted.

“These data and disparities shock the conscience,” King wrote.

King cited myriad research showing that physical punishment actually exacerbates poor behavior, rather than correcting it. Students who receive corporal punishment are more likely to be depressed or show antisocial behavior as adults.

One of the alternatives King proposed is restorative justice, which aims to address the root causes of disagreements and misbehavior through conversation rather than punishment. The practice has already gained steam in numerous Colorado schools, including Denver Public Schools and Aurora Public Schools.

“The most frequently cited rationale (for corporal punishment) tends to be tradition and concern about how schools can ensure safe and orderly environments,” he told reporters in a briefing on Monday. “… (But) there are smarter ways to achieve safe and supportive environments.”

King’s letter comes at a time when Colorado is grappling with its approach to disciplining preschoolers and early elementary students. Over the past two years, there’s been a statewide push to reduce suspensions and expulsions of young children, a practice that disproportionately affects boys of color. It’s likely there will be legislation on the topic during the 2017 session.

Disparities in discipline have been a rallying cry for the Obama administration, which has launched several initiatives aimed at decreasing racial disparities and increasing the use of alternatives to suspension, expulsion and corporal punishment.

It’s unclear how the administration of President-elect Donald Trump will approach the matter, but the head of the nation’s largest teachers union urged continued advocacy against corporal punishment, even as Obama’s administration ends.

“It does not matter who the secretary of education is. … This is a moral matter,” said Randi Weingarten, president of the American Federation of Teachers. “I don’t care if you are a Republican or a Democrat, a conservative or a progressive. …There is no earthly justification for paddling, caning or otherwise physically harming students in schools.”