A SCCRE study on the effectiveness of additional instructional time shows a causal impact of additional instruction time on student test scores, albeit in a limited extent. Moreover, additional instruction time significantly increases the within-school variance of subject-specific test scores.
Maria Alejandra Cattaneo, Chantal Oggenfuss, Stefan C. Wolter (2016):
The More, the Better? The Impact of Instructional Time on Student Performance.
IZA Discussion Paper 9797