The Life League has leafleted his neighbours, published the school's phone number and has urged people to ring up and protest.
Eddie de Middelaer, the head of Lutterworth grammar in Leicestershire, sent an open letter to 2,000 parents explaining why about two pupils a week have been given the pill over the past four years, a figure thought to be one of the highest in the country.
He said the school, which has a high number of pupils over 16, assessed each pupil asking for emergency contraception to "see if they are mature enough to use the service". The pill, he said, was offered by health professionals in the school's surgery.
"Pupils receive appropriate advice, counselling and support about all sexual issues - ranging from sexually transmitted infections to whether or not a sexual relationship is appropriate for them," Mr de Middelaer said in the letter. "The girls are encouraged to inform their parents, but it is a confidential service and that is one of the reasons it is so successful."
The Life League, founded in 1999, branded Mr de Middelaer "irresponsible" and accused him of "poisoning girls by making the morning-after pill available".
James Dowson, a spokesman for the league, said: "This guy seems to think there is nothing wrong with what he is doing. The school is not allowed to hand out pain killers but it's handing out these pills without parents' knowledge. We don't want to make life hell for Mr de Middelaer, but we want to make him see how serious this is."
Leicestershire police say they are "monitoring the situation".
Last year the group published on its website the home address of a gynaecology nurse working at Kings College Hospital, London, and also published details of a sex education teacher working at Woldingham school in Surrey.