Phoenix police say a parent notified officials at Shadow Mountain High near 30th Street and Shea on Thursday morning.
They say the unidentified boy didn't have any weapons on him when he went to school and his motive for the alleged threats isn't immediately clear.
Police say the teen's threats included creating his own bacteria of which he would be immune to.
But the school's principal says there were valid concerns for the students and staff.
Police say the boy could face threats charges along with disruption to an educational facility.
They say had no previous disciplinary or criminal issues.
Letter sent regarding the allegation:
Copyright 2015 The Associated Press modified.