While existing research supports that participation in high-school athletics is associated with better education and labour-market outcomes, the mechanisms through which these benefits accrue are not well established. We use data from a large public-school district to retrieve an estimate of the causal effect of high-school athletic participation on absenteeism. We show that active competition decreases absences, with most of the effect driven by reductions in unexcused absences – truancy among active male athletes declines significantly, with the effects larger in earlier grades and for black and Hispanic boys. Strong game-day effects are also evident, in both boys and girls, as truancy declines on game days are offset with higher rates of absenteeism the following day. Addressing the effects on academic performance, we find significant heterogeneity in the response to active athletic participation by race, gender and family structure, with boys not in dual-parent households exhibiting small academic improvements in semesters in which they experienced greater athletic participation.