Sheriff's deputies broke up a fight at San Lorenzo High School Monday morning.
School officials downplayed the incident as involving a small number of girls who had been friends but, for reasons as yet unknown, came to blows.
"We're doing an investigation to determine why these friendships went bad and why they did this at school," said Dr. Melanie Spears, directory of secondary education for the San Lorenzo School District.
A note posted on Facebook and Twitter Monday called the incident a "riot" between black and Latino girls.
But Spears said only a few girls were involved. And while some were black and at least one was a Latina, it is not clear that race or ethnicity had anything to do with the altercation, she said.
Sheriff's Department spokesman Sgt. J.D. Nelson said the incident involved several individual fights, and that no one was hurt or arrested.
A California Highway Patrol spokesman said CHP officers were initially called to help break up the fights.
But while they were still enroute, the sheriff's deputies brought the situation under control and so the CHP officers turned back before reaching the school, the spokesman said.
Spears said parents are picking up the students involved. Disciplinary action will be decided later.
She said the fight came as an unpleasant surprise to district officials who had seen a welcome decline in disciplinary incidents.