BackgroundAdolescent men have a vital yet neglected role in reducing unintended teenage pregnancy (UTP). There is a need for gender-sensitive educational interventions.ObjectivesTo determine the value and feasibility of conducting an effectiveness trial of theIf I Were JackRelationship and Sexuality Education (RSE) intervention in a convenience quota sample of post-primary schools in Northern Ireland. Secondary objectives were to assess acceptability to schools, pupils (male/female, aged 14–15 years) and parents/guardians; to identify optimal delivery structures and systems; to establish participation rates and reach, including equality of engagement of different socioeconomic and religious types; to assess trial recruitment and retention rates; to assess variation in normal RSE practice; to refine survey instruments; to assess differences in outcomes for male and female pupils; to identify potential effect sizes that might be detected in an effectiveness trial and estimate appropriate sample size for that trial; and to identify costs of delivery and pilot methods for assessing cost-effectiveness.DesignCluster randomised Phase II feasibility trial with an embedded process and economic evaluation.InterventionA teacher-delivered classroom-based RSE resource – an interactive video drama (IVD) with classroom materials, teacher training and an information session for parents – to immerse young people in a hypothetical scenario of Jack, a teenager whose girlfriend is unintentionally pregnant. It addresses gender inequalities in RSE by focusing on young men and is designed to increase intentions to avoid UTP by encouraging young people to delay sexual intercourse and to use contraception consistently in sexual relationships.Main outcome measuresAbstinence from sexual intercourse (delaying initiation of sex or returning to abstinence) or avoidance of unprotected sexual intercourse (consistent correct use of contraception). Secondary outcomes included Knowledge, Attitudes, Skills and Intentions.ResultsThe intervention proved acceptable to schools, pupils and parents, as evidenced through positive process evaluation. One minor refinement to the parental component was required, namely the replacement of the teacher-led face-to-face information session for parents by online videos designed to deliver the intervention to parents/guardians into their home. School recruitment was successful (target 25%, achieved 38%). No school dropped out. Pupil retention was successful (target 85%, achieved 93%). The between-group difference in incidence of unprotected sex of 1.3% (95% confidence interval 0.55% to 2.2%) by 9 months demonstrated an effect size consistent with those reported to have had meaningful impact on UTP rates (resulting in an achievable sample size of 66 schools at Phase III). Survey instruments showed high acceptability and reliability of measures (Cronbach’s alpha: 0.5–0.7). Economic evaluation at Phase III is feasible because it was possible to (1) identify costs of deliveringIf I Were Jack(mean cost per pupil, including training of teachers, was calculated as £13.66); and (2) develop a framework for assessing cost-effectiveness.ConclusionTrial methods were appropriate, and recruitment and retention of schools and pupils was satisfactory, successfully demonstrating all criteria for progression to a main trial. The perceived value of culture- and gender-sensitive public health interventions has been highlighted.Future workProgression to a Phase III effectiveness trial.Trial registrationCurrent Controlled Trials ISRCTN99459996.FundingThis project was funded by the NIHR Public Health Research programme and will be published in full inPublic Health Research; Vol. 5, No. 1. See the NIHR Journals Library website for further project information.