Objective − The objective of the research was to examine the mental health of Croatian high-school students before and during the COVID-19 pandemic with regard to gender, age, school, residence, family structure, and parents’ education.Materials and Methods − Total of 211 15-year-old first grade high-school students completed the YP-CORE questionnaire in the 2020, just before the lock-down enforced due to the COVID-19 pandemic. In 2022, now being 17-year-olds, 3rd-grade high-school students filled out the YP-CORE questionnaire once again during the COVID-19 pandemic. Socio-demographic data were collected from the participants’ health charts.Results − The median YP-CORE score significantly increased from 7 in 2020 to 11 in 2022. A significant increase in psychological distress was determined, from 11% of adolescents in the pre-pandemic period to 23% of adolescents in the pandemic period. Significantly higher rates were reported by girls in both periods. Students from the School of Applied Arts and Design reported psychological distress significantly more often than students from the Electrical Engineering and Traffic School and the Grammar School before and during the pandemic. Family structure was significantly associated with YP-CORE scores only in the pre-pandemic period.Conclusion − The study showed that the COVID-19 pandemic had negative impact on mental health of Croatian adolescents, with certain sociodemographic risk factors being associated with poorer mental health outcomes.