Pros to year round schooling include avoiding what educators call "summer learning loss", which is the well-documented phenomenon of students forgetting and regressing over the summer break. Another positive is that more students can be accommodated at one school. Most year round schools are based on an 8 weeks on, 3 weeks off alternating schedule, which still provides for family vacation time and may let families have a better chance to actually spend time together. Students are less likely to get bored during a 3 week break than during summer break, which is a long stretch for many.
Cons include the fact that older students cannot have a summer job to earn money; this is also true for the many teachers (like me) who work a second job in the summer. In areas where tourist dollars are important, students make up a very large fraction of the labor force, so the businesses who depend on student summer labor would suffer. Many students also use the summer break to pursue internships and special educational programs, which would not be possible if they went to school year round.
In many areas school buildings do not have air conditioning, which would be an added expense, both for installation and operation. Some have said that having school buildings sit empty in the summer is inefficient, but this does not take into account the fact that in most schools the summer vacation offers the maintenance staff to do major repairs without disrupting the educational process; again, working around this to accommodate year round school might have cost impacts.