All three victims were treated for bites and scratches at a local hospital, said district spokesman Will Ector, and all are undergoing rabies treatment as a precaution. They also were administered antibiotics, he said.

The attack occurred as a classroom of first-graders was preparing to go on a field trip, said Ector. Two parent chaperones were standing in the room when a squirrel ran in and then up the leg of one of the parents, he said.

"They were trying to get it off and another parent was trying to assist in getting it off," he said. "In the process, both were bitten. One was nipped on the fingertip and scratched on the arm, and the other parent was bitten on the arm."

Finally, the squirrel jumped off the mom, did a loop around the classroom, then ran out the door and jumped on an 11-year-old student walking by. She was bitten on the arm as well, Ector said.

San Jose Police spokesman Sgt. Nick Muyo said police received a call around 8:45 a.m. alerting them to the attack. The student and one of the parents were bitten hard enough to draw blood, he said.

They were taken to the hospital by ambulance after being treated by the school's health services, Ector said. The other parent drove herself to the hospital, he added.

After the attack, Ector said, the school went into a lockdown as a safety precaution. They called in an animal expert and have since set live traps around the campus.

"It's not a heavily infested area, it's not like there are squirrel everywhere," he said.

Ector acknowledged that even if a squirrel is caught, it will be hard to verify whether it's the right critter.

"Anything we catch we will turn over to the county for them to check," he said.

It's not the first aggressive squirrel the South Bay has seen lately. Since the fall, there have been problems at Mountain View's Cuesta Park, with at least four people suffering bites.

Animal control officials attributed the aggressive behavior to a preponderance of food being left around the park, which had emboldened the animals. In one case, a 4-year-old boy was bitten after a squirrel attempted to steal his muffin.

Earlier this year, officials began trapping and euthanizing the squirrels at Cuesta Park.

E-mail Marisa Lagos at mlagos@sfchronicle.com.