There is a lot to consider when you think about if a traditional school year or year-round schooling is best.  There are pros and cons to each one.

Let's start with the pros:

Consistency

When you think about consistency, it seems that a year-round school schedule would be better.  Students learning at a school with a traditional schedule have a long two month break in which knowledge is forgotten.  A child can work so hard during the school year to memorize something, such as the multiplication table, only to remember just a fraction of their facts after the summer break.  The student will, of course, memorize it again, but valuable school time is wasted in re-learning forgotten information.  Shorter three week breaks provide less time to forget.

More Vacations

While a year-round school may not have one long vacation, you will have several shorter vacations throughout the school year.  This could be to your advantage if you are able to travel to a vacation spot that is asking lower rates since it is not during the prime summer or winter season when most students are on break.  These vacation spots would also be much less crowded off-season.

Boredom Busting

Not every child is involved in camps, vacations, or other fun activities during the long summer vacation.  In fact, there are many students that, although they would never admit it, are bored and are actually more than ready for school to start in the fall.  A year-round school schedule does not allow enough time in the vacation for the children to become too bored.

More Students

A school following a year-round school schedule may be able to accommodate more students than a traditional school.  While some year-round schools will have all of the students on the same schedules; others will have the students rotate their vacations so that there is always a group of students on vacation while another group is in school.  This will allow a school to have more students and alleviate overcrowding and possibly even remove the necessity of building a new school.  This also means that teachers may have a higher yearly salary if they work throughout the year without a long summer break.

Now, on to the cons:

Long Vacations

Let's say that you are one of the many families that have relatives in another country.  Quite a few of these families with international ties will use the long summer vacation to spend a month or more with their family and friends in another country.  Considering how long the flights are and how much these flights cost, it is not terribly feasible to try to go on these international vacations during the short break provided by year-round schools.  While this may not affect the majority of the student population, it would be difficult for the families that would either need to keep their vacations short or take their children out of school; in which case those select students are placed at a disadvantage.

Off Schedule With Other School Systems

One major conflict a family attending year-round school would face is that it is not a very popular practice in most of the country.  This can make trying to coordinate with friends and family in other school systems a challenge.  Even colleges have fall and spring sessions with an option to take classes during the summer.  Those families with children on completely different school schedules would find it difficult to make plans.

Air Conditioning

Unfortunately, not every school is new or has even been built recently.  Some of these older school buildings are not even equipped with air conditioning.  A year-round school schedule would be nearly impossible to maintain during the summer if there are any affected schools without air conditioning and adding air conditioning is a very expensive addition to a school.

Summer Jobs

Some older students rely on getting a summer job to save up some extra money.  Without a long summer break, it would be impossible to get a job for just during three week breaks.  The student is then left with the choice of either not getting any job or trying to juggle a part-time job during the school year.  Neither of those options is a very convenient for most students.

Goodbye Summer Camps?

There are many wonderful summer camps for children that would be in danger of disappearing were schools to turn to year-round schooling.  These camp provide many opportunities that are usually out of reach to children during the school year.

Studies have been inconclusive as to which school schedule is most beneficial to students.  Year-round schooling continues to be a controversial subject that parents and educators will probably never all agree on because each person's perspective will affect whether the pros outweigh the cons.