However, teachers have found group activities make the task easier.
Trainee teachers from 10 Rajabhat universities are applying these techniques together with dialogue at secondary schools during pre-service teaching.
"Water exchange is an activity my secondary students like. They tell me it helps them understand clearly why promiscuous people are at risk of HIV/Aids," said Piyanan Maklohley, 24, a Phra Nakhon Si Ayutthaya Rajabhat University trainee teacher.
"I give them a bottle of clean water each. But, one bottle has a chemical substance added, which we can't see. It's meant to be HIV-infected blood.
"I let them exchange water, which some exchange with many students and others exchange with only one.
"After this we use another chemical to check whose water bottles have become infected. The second chemical turns the water violet. This means they have become infected. It is like blood testing," she said.
Piyanan said the exercise was always a big talking point among the students.
"I used to be shy talking about sex, but now I'm not. Sex education in this form helps me know more about it and understand things I never knew before.
"I can use what I learn in everyday life. It's very useful," Norathep Fungladda, 14, a ninth-grade student said.
The Rajabhat trainees are taught "sexuality education and learning design". It is a new course, adapted from the programme for appropriate technology in health, or Path.
Once complete the trainees are qualified and confident to teach teenagers sex education.
Almost 1,000 trainees have studied the three-credit, one-term course and 253 schools in 16 provinces have been the beneficiaries.
They learn 12 main topics using the activity method. These include sexuality, reproductive and sexual health, social values, gender issues, sexual violence, marriage and lifetime commitments and family.
The trainees teach sex education as a special class, as well as offering instruction in their majors.
Assist Prof Chintana Vechmee of the Phra Nakhon university is the coordinator of Teenpath. She said Path and the Rajabhat universities started the courses in schools because teenage students were embarrassed to discuss sex with their teachers and even parents.
Chintana said the idea grabbed the attention of visitors to the International Aids Conference in Mexico last month. It is one of the few practical projects in schools helping prevent transmission of the virus.
She's going to collect names of parents who want this format taught to their children to encourage other schools to become involved.
Next month Path will meet with teachers to draft policy proposals for sex education," she added.
Waranuch Chinvarasopak, a Path programme officer, said it wanted an Education Ministry commitment to provide sex education as a separate subject, and not simply include it in hygiene.
"Path will expand the project to other interested universities within three years," Waranuch said.