1 McGill University, Faculty of Education, Department of Educational and Counselling Psychology, 3700 McTavish, Montreal, Quebec H3A 1Y7, Canada. Electronic address: jessica.mettler@mail.mcgill.ca.
2 McGill University, Faculty of Education, Department of Educational and Counselling Psychology, 3700 McTavish, Montreal, Quebec H3A 1Y7, Canada. Electronic address: Stephanie.zito@mail.mcgill.ca.
3 McGill University, Faculty of Education, Department of Educational and Counselling Psychology, 3700 McTavish, Montreal, Quebec H3A 1Y7, Canada. Electronic address: Laurianne.bastien@mail.mcgill.ca.
4 Concordia University, Campus Wellness and Support Services, 1550 De Maisonneuve W, Montreal, Quebec H3G 1M8, Canada. Electronic address: Elana.bloom@concordia.ca.
5 McGill University, Faculty of Education, Department of Educational and Counselling Psychology, 3700 McTavish, Montreal, Quebec H3A 1Y7, Canada. Electronic address: Nancy.heath@mcgill.ca.

Given high levels of adolescent stress and educational institutions' key role in supporting students' mental health, mindfulness instruction is increasingly being implemented in schools. However, there is growing evidence adolescents find traditionally taught formal mindfulness (e.g., structured regular practice like meditation) challenging. Indeed, school-based studies report high levels of student non-compliance and lack of engagement with formal mindfulness strategies. Thus, informal mindfulness practices (e.g., unstructured brief moments integrated within daily routine) may be more accessible and developmentally appropriate for adolescents. Using a randomized experimental school-based design, this study sought to parse out the acceptability and effectiveness of formal and informal mindfulness for adolescents over time. Adolescents (n = 142; 73.9% female) were randomly assigned to a 4-week formal mindfulness, informal mindfulness, or comparison group and assessed on mental health, well-being, and educational outcomes. The informal mindfulness group (a) was more likely to report intending to frequently use the strategies (p = .025, Cramer's V = .262) and (b) reported increased dispositional mindfulness (i.e., general tendency to be mindful) from baseline to follow-up (p = .049, ?_p² = .034) which in turn mediated benefits on depression (indirect effect = -.15, 95% CI [-.31, -.03]), anxiety (indirect effect = -.21, 95% CI [-.36, -.06]), general stress (indirect effect = -.16, 95% CI [-.32, -.04]), school-related stress (indirect effect = -.15, 95% CI [-.28, -.05]), negative affect (indirect effect = -.17, 95% CI [-.35, -.04]), and attentional control (indirect effect = .07, 95% CI [.01, .13]). Thus, brief informal mindfulness strategies may be easier for students to use on a regular basis than formal mindfulness. Overall, these findings highlight the importance of going beyond a one-size-fits-all approach by offering accessible and engaging school-based mindfulness instruction to students. Recommendations for school psychologists seeking to teach mindfulness to adolescents are discussed, including the need to directly teach how to integrate informal mindfulness strategies in students' lives.