Adolescent sexuality is an important public health issue, as it affects them in many ways; they
risk contracting HIV and other sexually transmitted infections; may become pregnant leading to
school dropout and early marriage. The assessment of prevalence of sexual activities among
students, may guide policies and programmes aimed at reducing the risks of engaging in early
sex. The study used ex posto facto and descriptive research design. Target population was 20227
form three students in Nairobi and Busia Counties. Accessible population was 3568 form three
students in seven districts. Multi-stage, probability proportionate to size, stratified and simple
random sampling was used to select the participants. Findings of the study shows adolescents
engage in sexual activities at a very young age. They engage in sex activities with multiple
partners and majorities do not use contraceptives. Overall prevalence of sexual activities was
28.8% male and 14.6% female. Variables positively associated with sexual activities include age,
gender, school type and location. Any intervention aimed to promote students’ sexual health
should be designed with appreciation of factors associated with sexual activity in due
consideration.