They said the girl returned home from school yesterday, complaining of "uneasiness and vomiting".
Her parents rushed her to a private hospital where doctors confirmed that she was sexually assaulted, police added.
Deputy commissioner of police (North Bengaluru) Chetan Singh Rathore said medical tests had been conducted and a case under the Protection of Children From Sexual Offences (POCSO) Act registered against unidentified persons. "The girl told her parents she was assaulted inside the school but CCTV footage available from 9.30 am onwards donot corroborate her statement. We havedetained the security guards of the school and are questioningthem," Rathore said.