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KS3: Sexually Transmitted Infections

In this lesson plan, a classroom-based activity is used to demonstrate how easily sexually transmitted infections (STIs) can be transmitted. Using chlamydia as an example, this lesson helps students to understand how quickly STIs can spread and the potential consequences of infection.

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Learning objectives
All students will:
  • Understand that infection can be spread easily through sexual contact
  • Understand what students can do to protect themselves against STIs
  • Know that not everyone with an STI has symptoms
  • Understand how easily infections like chlamydia can spread among young people
Most students will:
  • Understand that non-barrier forms of birth control do not protect against STI
  • Begin to explore effective communication surrounding condom use
Background Information

STIs are infections contracted by having close sexual contact with someone who is already infected. Some STIs can be treated and cured with antibiotics, whereas others cannot. Many symptoms of incurable STIs can be treated to make them easier to live with. There are over 25 different STIs.

Anyone can contract an STI. Most people who contract an STI do not know that the person they had sexual contact with was infected. When discussing sexual health with students, it is important everyone feels comfortable, safe, and heard. Suggested ground rules are provided in the teacher guidance accompanying the key stage 3 pack.

In this lesson plan, students explore how quickly sexually transmitted infections spread and how this can be prevented through the use of condoms. By looking at chlamydia, students discuss how the infection works, what the symptoms can be, and how the infection can be prevented.

Main activity:
  • Test tube experiment. The KS4 main activity video included below is also applicable for the KS3 experiment.
Main Activity video
Extension activities:
  • If chlamydia could talk group work
  • Guest speaker
  • STI quiz
Curriculum links


  • Working scientifically


  • Reading
  • Writing


  • Health and prevention
  • Intimate and sexual relationships
  • Sexual health
Supporting Materials
Teacher sheets
KS3 STI teacher guidance
Test tube experiment - video transcript
Student worksheets
SW1 Test tube STI experiment worksheet
SW2 STI quiz
Student handouts
SH1 If chlamydia could talk