By Kevin Murphy
The Republican-controlled state General Assembly voted to override Democratic Governor Jay Nixon's veto of a bill that included the weapons provisions.
The law allows school districts to select teachers or other employees to carry a concealed weapon or pepper spray as "school protection officers" if they pass a training program and meet other requirements.
"This is just an option for school districts, it's not mandatory," state Senator Will Kraus, the bill sponsor, said on Thursday.
Kraus said teachers who have permits can already carry guns in Missouri schools but the new law sets broader training and safety requirements.
In his veto message in July, Nixon said he consistently opposed arming teachers as a means to keep schools safe. "It is simply the wrong approach, and one that I do not support."
Some opponents of the law are concerned that creating designated gun-carriers will lead to more weapons in schools and potential incidents.
A Utah elementary school teacher with a state permit to carry a concealed firearm in class shot herself in the leg on Thursday when the handgun went off, apparently by accident, while she was in a faculty restroom.
The measure comes amid a protracted national debate over the extent of gun restrictions after a series of deadly rampages in schools, movie theaters and other public places. Shootings at U.S. schools and universities in particular have prompted calls to supplement police and university security forces.
The measure also lowers the age for getting a concealed weapons permit in Missouri to 19 from 21 and prevents cities and counties from passing ordinances banning the open carry of firearms by people who have concealed weapons permits.
Kansas City Mayor Sly James said on Thursday that lawmakers erred in overriding Nixon's veto. The city council has passed an ordinance against open carry.
"This legislation is dangerous and unfortunate and the override puts political interests above the interests of people," James said in a statement.
The legislation takes effect in about 30 days.
(Additional reporting by Curtis Skinner; Editing by Eric Walsh)