Before I can answer that, I'd want to know:
How would foreign language, music, performance, service, athletic, and club opportunities change?
What is the tone of the middle school, one that is supportive of differences, works to teach another path through "mean girl" years, and works to develops an adolescent's sense of self?
What are the needs of your population? One for more differentiation that can handle multiple teachers and the organizational demands of middle school, or a general population that needs an extra year of nurturing with a smaller cohort and fewer adults to answer to?
What does the leadership of the elementary school and middle school look like? Is there a better fit with one principal than another?
