A 14-year-old boy was charged Monday with raping a 12-year-old girl in the stairwell of an El Cerrito middle school, and the principal and vice principal have been placed on administrative leave while police and school officials investigate the alleged crime.

El Cerrito police will focus their investigation on the alleged sexual assault, while West Contra Costa Unified School District officials look into how such a crime could have occurred on the Portola Middle School campus during the school day, officials said.

The boy allegedly raped the girl about 2:40 p.m. Thursday at the school at 1021 Navellier St., police Cmdr. Michael Regan said.

The stairwell was empty because class was in session, he said. The two students, who apparently know each other, had passes or reasons not to be in class, he said. He declined to discuss other details of the alleged assault.

Another student who entered the stairwell and witnessed the incident reported it to a school resources officer, Regan said.

Police arrested the suspect at his home in Richmond about 8 p.m. Thursday. He was booked into juvenile hall in Martinez and charged Monday with rape and false imprisonment.

Marin Trujillo, a spokesman for the school district, said principal Denise VanHook and assistant principal Matthew Burnham had been placed on paid administrative leave, in keeping with a district policy adopted after the recent gang rape outside Richmond High School's homecoming dance.

"The purpose is to allow an investigation to take place," Trujillo said.

Investigators will look into why the students were not in class, what plans existed to supervise them, and why those plans were not followed.

"We're shocked that this could have happened during school hours," Trujillo said.

VanHook, reached at home Monday night, said she could not comment, and that she had not been apprised of the investigation's findings or progress.

"I'm in the dark as much as you are," she said.