Background: Early studies indicate children's physical activity (PA) decreased during the pandemic but may vary by school format and season. We longitudinally assessed changes in PA and screentime (ST) behaviors throughout 1 year of the COVID-19 pandemic and according to changes in school format. Methods: Parents of children (5-18 years) completed a survey about school format, home characteristics, PA, and ST. Initial data were collected in November 2020 with follow-up in February, May, and August 2021. Physical activity and ST were compared by season and school format using a 4 (format) × 4 (season) repeated measures analysis of variance. Results: Children were active for at least 60 minutes/day less often in winter (2.4 ± 0.3 days; p < 0.001; n = 176) than in other seasons. Longitudinal analysis (n = 61) showed that PA was highest among students attending school fully in-person and lowest among students attending remotely, though school format differences were not significant. Non-school ST did not differ by season or school format. Implications for School Health Policy, Practice, and Equity: These results suggest that changes in school format influence students' lifestyle behaviors. Conclusions: The impact on children's lifestyle behaviors should be considered in planning for safe, in-person operation of schools in the event of future pandemics or natural disasters.