The COVID-19 pandemic (2020–2022) caused unprecedented disruptions worldwide, significantly challenging adolescents' emotional and academic adjustment. This thesis explored these impacts through four studies across multiple psychological, social, and academic outcomes, among adolescents of varying ages and across Nordic countries and South-Australia.

Study I examined the effects of the first school lockdown in 2020 on bullying victimization among Finnish adolescents, revealing a significant decrease in bullying rates during the lockdown and no increase in cyberbullying. Study II applied latent change score modeling to explore changes in anxiety levels among Finnish emerging adults who graduated in 2020, identifying marginal increases in anxiety and failing to find evidence of prior anxiety or loneliness predicting negative effects. Study III utilized longitudinal multi-cohort data following two age cohorts of South-Australian adolescents before and during the pandemic, and applied multigroup latent change curve modeling to assess the pandemic's impact on academic self-efficacy and cognitive reappraisal, finding minimal negative effects after disentangling developmental effects. Study IV analysed PISA 2018 and 2022 data on 15-year-old Nordic students to evaluate changes and social disparities in student well-being and in the association of well-being and academic performance, indicating stable latent well-being profiles and no significant pandemic-induced inequalities related to student well-being.

Applying a multidisciplinary resilience perspective, the findings contribute to understanding the varied effects of the pandemic on youth and challenge the prevailing public narratives of widespread negative impacts and exacerbated inequalities. Additionally, the thesis offers insights into methodological considerations for future research on population-level crisis impacts.