Moving to junior high is a big step and even as a kid you prepare and start over in a new environment, just like you would do in high school and again in college, then in jobs and so forth. Changing environments I believe is important for preparing kids, dealing with change, mental state/clearing the slate of the last school. Also kids get to be the big kids on campus at 6th grade, again at 8th or 9th then again at 12th grade, they also get to experience being the lower grade at a school a few times. These experiences I think just make people prepare/focus better. Change is good when it comes to environments of learning.