Looking back at my own junior high experience and comparing it to my kids' middle school experience, I think momof4 has hit the nail on the head. Junior high was about academics, and the adults trusted that the adolescents were cable enough to work through their personal growth issues on their personal time. Middle school seems to be about everything but academics.
In junior high, I had really solid instruction in writing,including extensive grammar instruction. We did science labs that required the exact same sort of write-up that my high school labs later required. We studied actual history and civics (as opposed to the nebulous social studies).
In middle school, my daughters don't write, they produce power point presentations. The few writing assignments are given as group projects - my oldest was recently assigned a group poem to write. While they do science experiments, these seem to be purely for demonstration value as the students are never required to record observations in any standardized form.
My middle school daughters frequently miss class in order to attend assemblies during which some nominally well-known person tells them how important it is to stay in school and get an education. The irony of cancelling class to have an athlete extol the virtue going to class seems lost on middle school administrators.