Year round school scheduling can increase a district's capacity without requiring a new building. Year round schools spend less for vandalism and theft. Students in year round schools are generally absent less often and have better behavior in school.
Shorter breaks mean better retention of knowledge and less reviews are needed.
Class sizes can be reduced. Winter family vacations can be scheduled without children missing school. Summer vacations may be interrupted. Teachers can't take on a summer job or classes. Parents may have difficulty securing daycare services.