Background <p>School engagement and mental health are frequently linked, but within-person associations over time are largely unstudied. Emerging statistical techniques can better gauge how longitudinal changes in school engagement or mental health influence individual-level outcomes.</p> Methods <p>Two cohorts of students (recruited in 2017 or 2018) in 5 Los Angeles high schools completed baseline surveys at the high school transition and 3 annual follow-up surveys through 11th grade. Random-intercept cross-lagged panel models explored the strength and directionality of associations between school engagement (Student Engagement Instrument) and mental health (Mental Health Inventory).</p> Results <p>Among 431 participants, we observed between-person and within-person correlations between school engagement and mental health. Autoregressive effects of school engagement and mental health on future levels of these variables were identified, with more consistent effects in late high school. A single cross-lagged effect from mental health to school engagement across the high school transition was identified.</p> Conclusion <p>Decreased school engagement may signal corresponding cross-sectional changes in mental health, presenting opportunities for monitoring and intervention. Changes in school engagement or mental health may influence later within-person changes in these constructs, but associations are likely weaker and less consistent than previously assumed based on techniques that conflate between-person and within-person effects.</p> Impact <p><UnorderedList Mark="Bullet"> <ItemContent> <p>This study assesses bidirectional and longitudinal relationships between school engagement and mental health among rising high school students.</p> </ItemContent> <ItemContent> <p>Both school engagement and mental health impacted future levels of these variables with stronger effects observed in the later high school years.</p> </ItemContent> <ItemContent> <p>Relationships between school engagement and mental health were less frequently observed than in prior studies, with an effect of mental health on future school engagement seen only across the high school transition.</p> </ItemContent> <ItemContent> <p>Interventions that promote both school engagement and mental health may be particularly beneficial, and the high school transition may be a promising time to leverage these.</p> </ItemContent> </UnorderedList></p>