First, congratulations on being a proactive member of your community.  There may be other considerations in the move your school district is planning. However, it is always good to gather as much information as possible to inform the decision.
 
There is a good deal of evidence that mixed-age grouping has a number of benefits.  Primarily, the benefits are seen in what is known as "pro-social" behavior: helping, sharing, and taking turns, for example.
 
Younger children will model their behavior on the more socialized older students and older students will generally be protective of younger students.
 
This benefit extends to at-risk students, as well, as they will be less likely to exhibit aggression towards the younger students and gain a self-esteem boost from being a role-model.
 
This dynamic also has academic benefits. Younger students receive tutoring from older children and the older students gain the "teacher" benefit: by explaining a concept to someone else, they better understand and internalize the concept themselves.
 
Below I've linked some articles with citations for studies that will help you make your case.
 
Most of these studies examine the benefits of grouping multiple age levels in the same classroom, however, it may be reasoned that these benefits will apply to mixed-age grouping within the school building to some extent.
 
Hope this helps!
 
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Candace Lindemann, Ed.M. - JustAsk Expert
Educational Consultant and Writer
http://CandaceLindemann.com