School schedules typically begin very early in the morning, which contrasts sharply with the naturally delayed chronotype of adolescents. Previous evidence shows that this misalignment leads to chronic sleep deprivation, social jetlag, and multiple negative outcomes, including poorer academic performance. Earlier studies by our group have documented these effects among Argentine students in the 1st and 5th years of secondary school. The aim of this research is to study the interaction between age and school shift on chronotype, social jetlag and sleep duration in 1st to 5th year secondary students, who were randomly assigned to morning, afternoon, or evening shifts at the start of secondary school. Our results indicate that as students progress through secondary school, their chronotype shifts later, while social jetlag and sleep deprivation on weekdays increases. Morning-attending students showed higher levels of social jetlag and sleep deprivation than their peers from afternoon and evening shifts. Importantly, most morning-attending students did not reach the recommended 8 hours of sleep at any age, even including naps. All these findings highlight the importance of considering the interplay between age and school timing for adolescents’ sleep habits in Argentina, given its association with performance, well-being, and educational outcomes, providing critical evidence to inform local public policies.