A law signed by South Dakota Governor Dennis Daugaard in March allowing teachers and school employees to carry a firearm on campus has taken effect.
The so-called "School Sentinel" bill is the first of its kind in the United States.
It was passed after the school shooting in Newtown, Connecticut last December that left 20 students and six teachers dead.
The law requires "sentinels" to be trained by police officers.
It was pitched as a way for small schools without nearby law enforcement to protect themselves against shooters or other dangers, according to South Dakota's Argus Leader newspaper.
The law was enacted despite opposition from the education community.