Yesterday, I found this report from the Education Policy Institue through several people posting it on social media. This evidence review explores in further detail how much time pupils typically spend in school in England and what the potential impacts of increasing this amount of time could be, particularly with respect to attainment.
The report identifies  five empirically proven common threads that consistently contribute to a more effective delivery of additional time across a wide set of studies:

• Extending core teaching hours has small effects, especially at the margin. Increasing the number of teaching hours within existing curricula for all students appears to be an intuitive approach to extending the school day, with studies indicating positive but modest effects. Cross-country comparisons, such as Lavy’s (2015) findings in relation to PISA scores and similar results in Denmark by Bingley et al. (2018), suggest that even substantial increases in instructional time may yield relatively small improvements. For instance, a Dutch programme with a five-hour weekly increase in instruction had no discernible impact on mathematics or language scores (Meyer and Kleveren, 2013), highlighting the challenges in achieving significant effects through extended school hours.

• After-school programmes are more effective when they are mandatory. In-school programmes, taking place before, during, or after the regular school day, have a larger impact than summer schools or weekend tutoring, highlighting the need for integration within the school day.

• Additional hours have a greater effect on test scores when pupils are instructed by the same teachers who conduct their regular classes. This emphasises the importance of specific training and experience and fostering a well-established connection between students and teachers.

• Non-academic extracurricular activities can have a positive impact on pupils’ academic achievements. Nonetheless, involvement in such activities has the potential to positively augment multiple dimensions of a pupil’s comprehensive well-being. Whilst a direct and swift improvement in academic performance might not be immediately evident, participation in activities such as sports clubs may contribute substantially to the holistic development of pupils.

• One-to-one or small group tuition is most effective but costly. Tutoring sessions are more likely to significantly and positively impact pupil academic outcomes when conducted in a one-on-one setting or in very small groups of two to three participants and a teacher. However, this is often prohibitively costly.