The current longitudinal study examined how between-person (BP) differences and within-person (WP) fluctuations in adolescents’ peer victimization and schooling format across ninth grade related to changes in their internalizing symptoms during the COVID-19 pandemic. Participants were 388 adolescents (61% female; Mage = 14.02) who completed three online surveys, administered 3 months apart, from November 2020 to May 2021. Multilevel modeling revealed BP (time-invariant) and WP (time-varying) effects of peer victimization and school instructional format (i.e., in-person; hybrid; online) on internalizing symptoms while accounting for potentially confounding demographic (e.g., gender) and contextual (e.g., COVID-19 positivity rates) factors. Results indicated that adolescents who experienced higher overall levels of peer victimization across the school year, compared to those who experienced lower victimization, reported more severe internalizing symptoms. Whereas relative WP increases in peer victimization predicted corresponding increases in adolescents’ depressive and somatic symptoms regardless of schooling format, WP increases in peer victimization only predicted elevated anxiety during months when students attended fully in-person, but not online, school. Adolescents who spent a greater proportion of their school year attending online school also reported less peer victimization across the year. Findings highlight WP fluctuations in the effects of peer victimization on internalizing and contextual variations depending on schooling format.