For the average kindergartener, the school day peaks around lunchtime. The whole class gets to eat together, then goes outside to play for a while before coming back inside to take a nap. Once they wake up, class resumes and the rest of the day breezes by.
School officials and parents aren't foolish enough to think these students should study during their lunch break and skip recess and naptime in order to cram in more work. The students wouldn't be able to concentrate under those conditions. They'd probably be terrors for the unfortunate teachers to handle, too.