A Phoenix police officer working as a school safety officer at Maryvale High School was alerted of an assault in one of the classrooms, police said.

Horne added that Maryvale has safety plans in place and a school safety officer (SSO) on campus during school hours. The difference between an SSO and a school resource officer (SRO) is that SROs are on school campuses full-time while SSOs are there one day a week, Horne said.

"Schools must be safe places,” State Superintendent of Public Instruction Tom Horne said, calling the incident a "terrible tragedy"