A 13-year-old girl and a 14-year-old boy were arrested on suspicion of battery on a schoolteacher. Both students were booked into juvenile hall in Martinez, Maples said.
Several other students have been identified as having been involved in the attack and Maples said there could potentially be more arrests.
The attack happened at about 12:50 p.m. Monday. Tiffany Carrico, a band teacher, had been assigned to keep students out of a particular classroom, police said.
When she denied access to a group of students, they allegedly threw rocks at her. As a larger group gathered, the students allegedly pushed Carrico to the ground, Maples said.
A vice principal at the school witnessed the attack and came to Carrico's aid. Carrico eventually escaped from the students and ran to her classroom, Maples said.
School district spokesman Marin Trujillo said Carrico has been placed on paid administrative leave while police and the school district investigate the incident.
Portola Middle School held a community meeting Tuesday night to talk to parents about safety, with officials from the Police Department and the West Contra Costa Unified School District present to answer questions, Maples said.
The school district agreed Tuesday to provide a school resource officer at the campus, and the officer was introduced to parents at Tuesday's meeting, Maples said.
He said El Cerrito police have had problems in the past with the administration of Portola Middle School.
"The principal and vice principal have shown a pattern of unwillingness to report criminal activity on the campus, and thought it was more appropriate to handle it with disciplinary actions at the school," Maples said.
The police chief has been meeting with school officials regularly, but the problem came to a head with Monday's attack, Maples said.
Portola Middle School Principal Denise Van Hook was in a meeting this morning and was not immediately available to comment.