Many students are part of clubs or sports that meet after school. If the school day were hours longer, likely these students would be exhausted by the time they could go to sports practices or attend their club. This could lead to lower productivity in clubs and lower performance in sports. It would also probably mean that fewer students would be involved in such activities.
If the school day were lengthened, likely some after-school activities would be greatly reduced or eliminated. It would become a long day for the children, and many parents would want them to be less involved so they could spend more time at home. Research shows that after-school activities are connected with improved academic performance in schools, so it is important for students to be involved in activities that are not work-related.
Another idea is to make what are now after-school activities part of the longer school day. As it stands, these programs, clubs and sports are optional. If the school day were lengthened, an activity could be required. Children could choose their activity -- a sport, a club or after-school academic help -- but they'd be required to stay at school. This would increase participation and possibly academic performance as well.
In any educational system, it is important that students are able to go home and relax and have time to do their homework, as well as get a lot of rest. With a longer school day and after-school activities still taking place, it is likely that by the time students got home, they would be too tired to do their best on homework. The stated purpose of lengthening the school day is to close the education gap, but if students are too tired to work at home, it defeats the whole purpose.