Worldwide, schools start very early in the morning, while adolescents exhibit very late chronotypes. As a consequence, both sleep and performance are negatively affected. International studies have shown that delaying School Start Times (SSTs) has positive effects on adolescents’ outcomes, but no evidence is available from Argentina. In this study, we conducted a before/after intervention assessing the effects of a 1-hour SST delay in the Morning shift (with the Afternoon shift as a control group) of a small school. All students completed several cognitive tasks at the first school hour of each shift, before and after the intervention. Our results showed that a 1-hour delay was associated with higher cognitive performance in Morning students. For example, we observed an increased number of trials and shorter response times in a Math addition task and in two different Go/No-Go tests (figures and letters). These findings highlight the benefits of later morning SSTs during adolescence, suggesting that longer sleep duration and/or performing later in the morning improve students’ performance, particularly in Argentina where chronotypes are especially late. Importantly, this is the first study to evaluate an SST delay in Argentina, providing local empirical evidence that can inform evidence-based educational policies aimed at improving adolescent well-being, health, and performance outcomes with potential long-term societal and economic impact.