Evidence suggests disclosing an autism diagnosis is associated with reduced stigmatization for autistic adults. However, it is unknown whether this is true for autistic adolescents. We used a vignette-and-questionnaire design to study stigmatizing attitudes with adolescents (aged 11–12 and 14–16 years, total N = 250) in a UK school. We investigated the effect of disclosing that a fictional adolescent had an autism diagnosis on stigmatizing attitudes of peers by testing the effect of disclosure of diagnosis on the social and emotional distance pupils wanted to maintain from the autistic adolescent. We also tested the effect of disclosure on peers’ assessment of the adolescent’s responsibility for their own behaviour. We checked to see if the effects were moderated by gender and age-group. Disclosing autism did not affect the social and emotional distance peers wanted to maintain from the autistic adolescent, but was associated with significant reduction in personal responsibility attributed to the adolescent’s behaviour. Boys attributed more personal responsibility to the autistic adolescent than girls, but this gender effect was reduced when autism was disclosed. These findings suggest that disclosing autism to other pupils may be of limited use in reducing stigmatization by peers in UK schools.