The survey, conducted by the university's health service unit, interviewed nearly 1,000 final year students aged between 18 and 22, and found that students from boys' or girls' schools had had sex six times more than co-ed students.

Among the interviewees, 480 had studied in co-ed schools, while 448 were in boys' or girls' schools either for their whole secondary education or some years.

The proportion of students having sex in boys' or girls' schools was six times more than those studied in co-educational schools. The former was 13.6 per cent while the latter was 8.4 per cent.

Dr Alexander Lo Chi-wah, clinical psychologist and head of Baptist College's careers and counselling services, advised students to enrol in co-ed schools so as to have a normal exposure to both sexes.

''Students in boys' or girls' schools have fewer opportunities to talk and play with the opposite sex, where they have to spend nearly eight hours a day in schools to face only students of the same sex,'' he said.

''But interaction with the opposite sex is a must for the growth of a healthy adolescent. Being 'isolated' in schools that lack contact with the opposite sex, these students are more curious to their male or female counterparts.

''Some of them easily fall in love and have sex when they make friends with the opposite sex.'' Dr Lo, who has done research on tertiary students' sexual behaviour and will head the Hongkong Tertiary Institutions Health Care Working Group, also warned of student tendency in exclusively boys or girls schools to turn towards homosexuality.

''Without enough interaction, they may find the opposite sex difficult to understand and resort to homosexuality instead. Many overseas studies have indicated this problem.'' Sister Flora Chu Man-pok, principal of Our Lady's College, admitted some of her students were quite ignorant about the opposite sex because of the lack of interaction.

''The girls seem to be very curious about their male counterparts. Some of them scream, laugh or speak loudly when they meet boys at the school gate. They don't know how to deal with them,''she said.

''Some students like to approach male teachers more than female in a bid to know more about the opposite sex,'' she said.

The Senior Sister of HKU's health service unit, Ms Lydia Ling Chi-ching, who helped conduct the survey, said the reasons for students in boys' or girls' schools having more sex were not studied.

But she agreed that it might be due to a lack of contact with the opposite sex and insufficient sex knowledge.