Making informed health decisions requires knowledge and skills in appraising health claims, and teaching adolescents these skills may prepare them for future decision-making. This cluster randomized trial evaluated the effectiveness of an educational intervention on students’ ability to identify and appraise health claims. Nine Australian high schools (4 control and 5 intervention) were recruited, comprising 974 students (382 control and 592 intervention) in Grades 7–10. Intervention impact was evaluated through baseline and follow-up evaluation. Follow-up mean scores on questions (maximum score of 25) from the Claim Evaluation Tools database (primary outcome) showed minimal between-group difference (intervention versus control: 14.4 versus 13.6; difference 0.8, 95% confidence interval [CI] −1.6 to 3.1; P = 0.52). Change scores were only slightly higher in the intervention group (difference 1.2 [95% CI −0.7 to 3.1; P = 0.21]). Between-group differences for secondary outcomes were also minimal. Most intervention group students ‘trusted’ and ‘liked’ the programme and found the content ‘easy’ and ‘helpful’. Most teacher feedback was positive, some noting challenges of covering content in allocated time and maintaining student engagement. It is unlikely that the assessed educational intervention had a large effect. Future research priorities are suggested.