"The bell had not rang, roll had not been taken, when the shots were heard," he said.
Staffers immediately began to lock down the campus, rushing students into classrooms and under desks when the gunman came around the corner toward the school, Fitzpatrick said at a press conference Tuesday.
The gunman crashed through the front gates of the school in a white pickup truck traveling at high speed, he said. Authorities say this was part of a larger rampage through the rural community in Northern California that left five dead and 10 wounded.
The man came out of the truck with a semiautomatic rifle and ran into the center of the school's quad and began firing at windows and walls as staffers, including the school's custodian, rushed students into classrooms under gunfire.
One student was shot in a classroom while under a desk, Fitzpatrick said. That student was said to be stable.
"The shooting came from outside; the shooter was not in the classroom, so it went through the wall," he said.
At that time the students and staff were secured behind locked doors and under desks.
"All of the staff were absolutely heroic in making sure that students were getting into the classrooms as shots were being fired," Fitzpatrick said. "This was a question of minutes."
Fitzpatrick said staffers described the scene as "horrific" with multiple rounds fired and multiple high-capacity magazines found at the school.
"It was a very, very difficult and terrifying situation which could have gone a lot worse," he said.
Fitzpatrick said the school of about 100 students will be closed for the week for repairs. Students were driven out of school after the shooting in a school bus to a local community center.
Counselors were present on campus for students, staff and parents.