Do not increase the school day, reduce absences instead

this was a very interesting paper.

While instructional time is viewed as crucial to learning, little is known about the effectiveness of reducing absences relative to increasing the number of school days. In this regard, this paper jointly estimates the effect of absences and length of the school calendar on test score performance. Using administrative data from North Carolina public schools, we exploit a state policy that provides variation in the number of days prior to standardized testing and find substantial differences between these effects. Extending the school calendar by ten days increases math and reading test scores by only 0.8% and 0.2% of a standard deviation, respectively; a similar reduction in absences would lead to gains of 5.8% and 3% in math and reading. We perform a number of robustness checks including utilizing u data to instrument for absences, family-year fixed effects, separating excused and unexcused absences, and controlling for a contemporaneous measure of student disengagement. Our results are robust to these alternative specifications. In addition, our findings indicate considerable heterogeneity across student ability, suggesting that targeting absenteeism among low performing students could aid in narrowing current gaps in performance.

so its not a question of increasing the school year, its a question of using the existing school year properly. if you are encouraging or condoning absenteeism, then the pupils dont take the lessons on board and do not have the discipline to handle the concepts. Quite an important finding, specially given the propensity of politicians to push for longer school years. See here for a UK example.