I like this piece in Slate on Paul Cuffee Middle School, a charter school right here in Providence. Most of what I know about child development seems to suggest that middle schools are sort of ridiculous. At the moment children are looking for role models and close relationships with adults (and not just the kids around them), we decide that kids should have many teachers, teachers should have higher loads, and the kids stay consistent while the adults change constantly.
In many ways, the elementary school model works better for middle school students and vice versa.
Anyway, some research showing K-8 schools have a built-in advantage against the traditional middle school:
The Middle School Plunge Stuck in the Middle 1