Investigators arrested the boys Monday at Clear Lake Middle School, where they are students, sheriff's Sgt. Candi Baker said.
The threat was aimed at Ranum High School, Baker said. She said investigators believed the threat was credible, but she declined to say why or what the motive might have been.
The boys were being held in a juvenile detention facility.
Last Friday was the eighth anniversary of the Columbine High School massacre just south of Denver in Littleton. Ranum High School is the suburbs north of Denver.