There is a new Best Evidence in Brief with among others, this study:
A study published in Economics of Education Review looks at the evidence from the extended school day (ESD) program in Florida to determine whether students benefit from longer school days.In 2012, Florida introduced the ESD program, increasing the length of the school day by an hour in the lowest-performing elementary schools in order to provide additional reading lessons. The lessons had to be based on research, adapted for student ability, and include phonemic awareness, phonics, fluency, vocabulary, and comprehension. Schools were selected using school-level reading accountability measures. For this study, David Figlio and colleagues looked at reading scores for all students in Florida between grades 3 and 10 using school administrative data from 2005-06 and 2012-13, and employed a regression discontinuity design to estimate the effect of lengthening the school day, looking at the different performance of schools either side of the cut-off point.Results indicated that the additional one hour of reading lessons had a positive effect on students’ reading achievement. ESD schools showed an improvement of +0.05 standard deviations on reading test scores in the first year. The annual cost of the ESD program was $300,000-$400,000 per school, or $800 per student.