This is why Ms Harrison, school counsellor at Lord Williams's comprehensive, in Thame, Oxfordshire, welcomed the decision to provide emergency contraception on school premises.
"Buying it over the counter is the worst option," she said. "There is no aftercare, no discussion. Teenagers need to be able to think the situation through carefully, when they are not panicking.
"Emergency contraception is not always a good thing, but it is often the lesser of many evils. At school the pupil can come to me afterwards to discuss relationships and broader issues, such as sexually-transmitted diseases."
Lord Williams's has participated in the Bodyzone programme, a scheme run by Oxfordshire health authority in 15 schools, since 1997. A family-planning nurse provides free contraception and counselling. Most of the schools have given out the morning-after pill since Bodyzone was set up.
Jane Bullock, chair of the Bodyzone steering group, said: "Statistics show that young people often don't use contraception at first contact. It's very gratifying that we have young couples coming to us before becoming sexually active, to discuss how to protect themselves. But sometimes you have to shut the stable door."
As soon as they start school, Lord Williams's pupils are made aware of Bodyzone's existence. In Year 9, there is a series of profile-raising workshops, tackling issues such as Aids and smoking.
The clinic is held in the school library, so pupils can drop in without arousing suspicion.
"Teenagers think they will be told off for not using contraception," said Ms Harrison. "But here, they know they will be treated with respect and sympathy. It is an easy way into the medical machinery."
Chris Harris, deputy head, said pupils benefit from ready access to a non-judgemental service. "Rather than taking an ethical stance on how early students are sexually active, we just have to accept that they are," he said. "School is a familiar place. Even the most sceptical students have a degree of confidence in the level of care and support they will receive."