Woldingham school contacted the police after receiving emails and letters attacking its decision to teach contraception to 14 and15-year-olds. One accused the head of "child abuse".
The 500-pupil school is run by an order of nuns known as the Society of the Sacred Heart. Boarding fees are pound;21,000 a year for 11 to 18-year-olds.
The emails were received after the school featured in the online newsletter of the anti-abortion group UK LifeLeague. The group claims to have 17,000 subscribers in Britain and the United States.