Background Programs for mental health promotion and prevention of nonsuicidal self-injury (NSSI) in schools have gained increased focus during the last decades, but less is known about adolescents’ experiences of such interventions.

Methods A whole-school preventive intervention targeting mental health and NSSI was delivered to six secondary schools. Adolescents participated in the Youth Aware of Mental Health program combined with an NSSI-focused psychoeducation module. Caregivers and teachers were given online psychoeducation on NSSI, and school health care staff were given a workshop on self-injury. Eleven group interviews (n = 65 participants) were conducted with adolescents (ages 13–15 years, 65% females) exploring participant experiences. Interviews were analyzed using thematic analysis and interpreted in light of a biopsychosocial understanding of adolescence.

Results The analysis generated two main themes. The first theme, Mental health in the context of adolescence, centered around adolescents’ conception of mental health, after having taken part in the intervention, framed in a context of coping with external stressors. The dilemma of autonomy versus help-seeking was also identified as part of the first main theme. The second theme, The who, when, what, and how of the intervention, described adolescents’ experiences of the intervention. This theme included increased awareness and knowledge of mental ill-health. The participants generally agreed that the topics included are important to adolescents and emphasized that the content needs to be relatable. Several factors that influence how a school-based program is received by adolescents were identified, such as who should be targeted and when. Adolescents also identified challenges and gave recommendations for future similar projects.

Conclusions Adolescents generally perceived addressing mental health and NSSI in schools as important. Help-seeking initiatives need to be balanced against adolescents’ need for autonomy when planning mental health prevention and intervention.