Students have fewer non-academic learning opportunities
If a student goes to school year round, this leaves much less time open for them to have extracurricular activities or non-academic hobbies. Students would be required to spend their year on homework and studying, with less consolidated time to spend on cultivating their personal interests.
During the school year, students have fewer opportunities to participate in extracurricular activities.  These activities can include study abroad, sports, music lessons, and others. If school is extended to be in session year round, then students would not have the access to pursue their personal goals and hobbies.  Over time, this constant schedule can make students feel repressed and grow to hate school, in turn, making them perform less successfully or get worse grades. Students in higher grades would also have less availability to work part-time. Forming well-rounded citizens requires participation in more than one activity. School year round prohibits well-roundedness by forcing students to focus on school for the near entirety of their young lives.  If parents do decide to put children into extra programs, their schedules are very packed and the parents have scheduling difficulties. The child also suffers because they never get to properly relax. Non-academic growth is vital to a person's success in life and year round schooling threatens that success.
Year round schools offer plenty of services for students to stay engaged.  Year round schools have clubs and other organizations that students may participate in. In year round schools, participation levels are higher than traditional schooling systems because the students are closer to one another and are far more familiar and comfortable in their schooling environment. 
Rejecting the premises