Officials say the fire broke out in the cafeteria building just before 2:30 a.m. Smoke and the flames were visible from miles away.
"We heard an explosion and we checked out the window and it was on fire," said Ana Hernandez of El Monte.
As flames shot into the sky, the roof on the building came crashing to the ground. Approximately 155 firefighters battled the flames throughout the morning, while students rushed over to check on the situation even though school was not in session due to Martin Luther King, Jr. Day.
Crews knocked down the fire around 6:30 a.m. They were able to keep the flames from spreading to other buildings, authorities said, but the building that burned wasn't just the cafeteria. It also housed the maintenance office, student store and ASB office.
"The commission room had everything in there since 1971. We have a lot of memories, records, photos," said student Sergio Mora, Jr.
One firefighter suffered a minor back strain. No other injuries were reported.
The fire appears to have started in the attic, but no cause has been identified yet. Despite the damage, the school will go back in session Tuesday for students.