BACKGROUND: Since the introduction of the Teenage Pregnancy Strategy (TPS), England's under-18 conception rate has fallen by 55%, but a continued focus on prevention is needed to maintain and accelerate progress. The teenage birth rate remains higher in the UK than comparable Western European countries. Previous trials indicate that school-based social marketing interventions are a promising approach to addressing teenage pregnancy and improving sexual health. Such interventions are yet to be trialled in the UK. This study aims to optimise and establish the feasibility and acceptability of one such intervention: Positive Choices. METHODS: Design: Optimisation, feasibility testing and pilot cluster randomised trial.Interventions: The Positive Choices intervention comprises a student needs survey, a student/staff led School Health Promotion Council (SHPC), a classroom curriculum for year nine students covering social and emotional skills and sex education, student-led social marketing activities, parent information and a review of school sexual health services.Systematic optimisation of Positive Choices will be carried out with the National Children's Bureau Sex Education Forum (NCB SEF), one state secondary school in England and other youth and policy stakeholders.Feasibility testing will involve the same state secondary school and will assess progression criteria to advance to the pilot cluster RCT.Pilot cluster RCT with integral process evaluation will involve six different state secondary schools (four interventions and two controls) and will assess the feasibility and utility of progressing to a full effectiveness trial.The following outcome measures will be trialled as part of the pilot:Self-reported pregnancy and unintended pregnancy (initiation of pregnancy for boys) and sexually transmitted infections,Age of sexual debut, number of sexual partners, use of contraception at first and last sex and non-volitional sexEducational attainmentThe feasibility of linking administrative data on births and termination to self-report survey data to measure our primary outcome (unintended teenage pregnancy) will also be tested. DISCUSSION: This will be the first UK-based pilot trial of a school-wide social marketing intervention to reduce unintended teenage pregnancy and improve sexual health. If this study indicates feasibility and acceptability of the optimised Positive Choices intervention in English secondary schools, plans will be initiated for a phase III trial and economic evaluation of the intervention. TRIAL REGISTRATION: ISRCTN registry (ISCTN12524938. Registered 03/07/2017).