All three suffered wounds that are not believed to be life threatening, police said.

One of the victims was a summer school student at International Studies Academy High School, said Gentle Blythe, a spokeswoman for the San Francisco Unified School District. A bullet or rock apparently grazed the student, who went home and then to San Francisco General Hospital.

Police arrested a man and recovered two guns at the scene after the 1:20 p.m. shooting at 18th and De Haro streets, police Sgt. Lyn Tomioka said. Investigators are trying to determine if there were other shooters.

The name of the man who was arrested has not been released.

The shooter or shooters apparently got out of a car down the block from the school's main entrance before opening fire. Students who were leaving the school ran back inside, and the building was put under lockdown, Blythe said.

District officials said they had received no indications there would be trouble after school. Four hundred students from high schools around the city attend summer school at International Studies Academy.

Chronicle staff writer Nanette Asimov contributed to this report. E-mail Jill Tucker at jtucker@sfchronicle.com.

This article appeared on page B - 1 of the San Francisco Chronicle