Perfectionism has adverse impacts on mental health and academic outcomes. We evaluated a 5-lesson classroom intervention for young adolescents delivered by teachers for impact on perfectionism, well-being, self-compassion, academic motivation and negative affect, at post-intervention and 3-month follow-up. Classes (N = 636 students, Mage = 13.68, SD = 0.60) were randomized to intervention (n = 343) or classes as usual (n = 293). Data were analyzed using linear mixed models adjusted for baseline observation and clustering. At post-intervention no differences were found between the groups. At 3-month follow-up, anxiety showed a significant increase in the control group with no increase in the intervention group (d = 0.23; 95% CI: 0.05, 0.40); females in the control group had a significant decrease in well-being from post-intervention to 3 month follow-up compared to the intervention group (d = 0.33; 95% CI: 0.08, 0.58); students with high levels of perfectionistic concerns in the intervention had significantly lower perfectionistic strivings at 3 month follow-up than the control group ((d = 0.34, 95% CI: 0.19, 0.49). This intervention shows promising results when delivered by teachers. Australian New Zealand Clinical Trials Registry (ANZCTR) Trial Number: ACTRN12621000457842 (April 19, 2021).