AbstractMost studies have shown a decline in the adherence to 24-Hour Movement Guidelines because of Covid-19 lockdown. However, there is little evidence regarding changes 1-year after the pandemic in these guidelines and their possible impact on academic performance. The study aims were: (1) to examine the possible changes in 24-Hour Movement Guidelines for youth (i.e., at least 60 min per day of moderate-to-vigorous physical activity, ≤ 2 h per day of recreational screen time, and 9 to 11 h of sleep per day for children and 8 to 10 h for adolescents) before and after 1-year into the Covid-19 pandemic, and (2) to examine the possible changes in the relationship between 24-Hour Movement Behaviours (physical activity, screen time, and sleep duration) and academic performance before and after 1-year into the Covid-19. This is a repeated cross-sectional study in two different samples of young Spanish at different times. Firstly, a total of 844 students (13.12 ± 0.86; 42.7% girls) completed a series of valid and reliable questionnaires about physical activity levels, recreational screen time, sleep duration and academic performance before Covid-19 pandemic (March to June 2018). Secondly, a different sample of 501 students (14.39 ± 1.16; 55.3% girls) completed the same questionnaires 1-year after Covid-19 pandemic (February to March 2021). Adherence to the three 24-Hour Movement Guidelines was significantly lower 1-year after into the Covid-19 pandemic (0.2%) than before the pandemic (3.3%), while adherence to none of these three recommendations was significantly higher 1-year after the Covid-19 pandemic (66.3%) than before the pandemic (28.9%). The positive relationship between physical activity levels and academic performance was no longer significant after 1-year into Covid-19 pandemic (β = − 0.26;p < 0.001). 1-year after Covid-19 pandemic, the relationship between recreational screen time (β = − 0.05;p > 0.05) and sleep duration (β = 0.05;p < 0.001) with academic performance did not change compared to pre-pandemic. The results suggest that 24-Hour Movement Behaviours have worsened among young people 1-year after Covid-19 pandemic compared to pre-pandemic period. Moreover, the physical activity benefits associated in terms of academic performance seem to have disappeared because of the Covid-19 pandemic. Therefore, there is a public health problem that requires priority and coordinated action by schools, policy makers, and researchers to mitigate the adverse effects of the pandemic on 24-Hour Movement Behaviours.