Maricopa County Sheriff Joe Arpaio told KTVK-TV he has the authority to mobilise private citizens to fight crime.
He says he does not plan to put posse members inside Phoenix-area schools but will have them posted around the perimeters.
Mr Arpaio is known as one of the nation's most high-profile supporters of a strict US-Mexico border policy.
His plan comes after two other Arizona officials announced ideas for school security.
Attorney General Tom Horne proposed firearms training for one person in each school, and Pinal County Sheriff Paul Babeu proposed training multiple educators per school to carry guns.
Twenty children and six adults were killed at Sandy Hook elementary school on December 14 in America's worst school shooting.
www.mcso.org (Maricopa County Sheriff's Office)