South Dakota Governor Dennis Daugaard has signed a bill allowing the state's school districts to arm teachers and other personnel with guns.
Supporters say the so-called sentinels could help prevent tragedies such as the Connecticut school shooting in December. The law will go into effect July 1.
The measure reflects a growing divide in the US between those like President Barack Obama who believe guns need to be more strictly regulated and supporters of the powerful gun lobby the National Rifle Association which maintains that more guns keep people safer.
Several representatives of school boards, school administrators and teachers opposed the bill during committee testimony last month.
They said the measure could make schools more dangerous, lead to accidental shootings and put guns in the hands of people who are not adequately trained to shoot in emergency situations.
But main bill sponsor Rep Scott Craig said earlier this week that he has received messages from a growing number of school board members and administrators who back it.
Craig said rural districts do not have the money to hire full-time law officers, so they are interested in arming teachers or volunteers.
The measure does not force a district to arm its teachers and would not force teachers to carry a gun.
On Monday, the South Dakota House voted 40-19 to accept the Senate version of the bill, which added a requirement that a school district must decide in a public meeting whether to arm teachers and others. Another Senate amendment allowed school district residents to push a school board's decision to a public vote.