class time affects student test scores
Schools that cancel class for unscheduled purposes, such as snow days, can see a drop in student proficiency, according to findings by University of Maryland researchers. So far, however, there's been little research on whether additional class time can boost scores even though extended-learning initiatives are becoming more widespread.
Marcotte, Dave E. and Hemelt, Steven W.,Unscheduled School Closings and Student Performance(July 2007). IZA Discussion Paper No. 2923
Available at SSRN: http://ssrn.com/abstract=1001409