Internationally, efficacy studies of school-based child sexual abuse prevention programs display a series of methodological shortcomings. Few studies include adolescent participants, recording of disclosures has been inconsistent, and no studies to date have assessed presenter adherence to program protocols or summated the costs of program implementation. A pretest-posttest waitlist control design was used to evaluate the Tweenees program delivered to grade 6 (n?=?88) and grade 7/8 students (n?=?117) compared to a control (n?=?185). Outcome measures included a knowledge/skills questionnaire, systematic coding of disclosures, and video interaction analysis of lessons. Costs were calculated per student, class, and school. Adolescents made small knowledge and skills gains indicating a program ceiling effect. Implementation analysis suggests low levels of adult control facilitated disclosures. Program costs were relatively inexpensive. Recommendations are made for future research.