The Hercules Middle-High School campus is seen in Hercules, California. Police are investigating an attack against a transgender teen in one of the school bathrooms. Picture: Jeff Chiu

San Francisco - Police on Tuesday were investigating reports of a brutal assault of a transgender teenager who was using a high school bathroom in California, where a new law allows students in public schools to use the bathrooms that match the gender with which they identify.

The 15-year-old student told officers he had opted to use the boy's bathroom at a school in a San Francisco suburb on Monday and was leaving it when three teenage boys pushed him inside a handicapped stall and physically and sexually assaulted him, police said.

The teen was taken to hospital for treatment.

Authorities said he was released on Monday evening and went home to his parents.

“He walked himself to the health centre, and was obviously very upset when he talked to officers,” Hercules police Detective Connie Van Putten said.

“He is doing as well as can be expected under the circumstances.”

Van Putten said the attackers reportedly made disparaging remarks to the student, allowing police to treat the incident as a hate crime.

Detectives were interviewing students on the campus north-east of San Francisco.

No suspects had been identified. - Sapa-AP