Hi Jennie, About thirty years ago, I had the opportunity to be in on the first wave of Middle School in Maryland. The main difference in philosophy is that while kids are in transition, they need some of the kind of support of Elementary programming to more gradually prepare for the big, impersonal world of High School.
Most Middle schools believe in teams of teachers who can be more attuned to individual students.
Unfortunately, philosophy frequently takes second place to convenience, expediency, and economics. I think it is very sad.