In tackling the widespread problem of bullying victimisation, researchers have acknowledged the value of focusing on changing bullying-related beliefs and using peer-based interventions. In three studies (N = 419, 237 intervention and 182 controls), we tested the effectiveness of the CATZ cross-age teaching programme by inviting small groups of 11-year-olds to incorporate information supporting positive beliefs (concerning non-physical forms of bullying, the value of disclosing being bullied to adults, and helping victims) into a lesson they devised for themselves and to deliver that to small groups of 9-year-olds. Specifically, we examined if the intervention would promote that (i) non-physical forms of bullying are unacceptable (study 1), (ii) disclosing bullying to adults and getting the right kind of help have value and importance (study 2), and (iii) victims can be assisted in safe ways (study 3). Self-reports of nine specific aspects of these beliefs were collected from CATZ tutors and age-matched controls prior to and following the intervention, and at five-week follow-up in one study, using both open and closed questions. Results indicated significant positive effects of CATZ on all nine outcome variables, with mostly medium and high effect sizes. These findings support the use of CATZ to foster positive anti-bullying beliefs, and issues related to its wider uptake are discussed.