Background: The COVID-19 pandemic negatively affected adolescent mental health due to school closures, isolation, family loss/hardships, and reduced health care access. Methods: We compared adolescent mental health in Rhode Island before versus during the pandemic, separately among middle and high schoolers. This serial cross-sectional study used Youth Risk Behavior Survey data from 2019 and 2021 (N = 7403). Multivariable logistic regression models estimated the association between year and mental health status, adjusting for sociodemographics. Results: Middle schoolers in 2021 had higher odds of ever seriously considering suicide (22.6% vs 16.7%) and ever attempting suicide (9.3% vs 6.1%) compared to 2019. Among high schoolers, those in 2021 had higher odds of experiencing persistent sadness/hopelessness in the past year (37.4% vs 32.0%). However, high schoolers in 2019 and 2021 had similar odds of considering suicide in the past year, while those in 2021 had lower odds of having attempted suicide in the past year (8.5% vs 14.6%). Conclusion: The COVID-19 pandemic may have worsened multiple aspects of adolescent mental health in Rhode Island, particularly among middle schoolers. Implications for school health policy, practice, and equity: Promoting school connectedness, creating supportive environments, and diversifying the mental health workforce may help overcome adverse pandemic effects.