I went to a K-8 school growing up. Thus, when I went to middle school, I simply moved down a floor in the building and was basically with the same group of kids I had been with in the elementary grades.
I was apparently an exception.
Most elementary students will go from small, highly structured schools in which they spend most of their day with the same teacher and classmates to much (in some cases, much much much) larger middle or junior high schools. Some of these middle schools will continue with a semblance of the elementary model in that the students move together with a "team" of teachers (called teaming) who will teach the students separately by subject, but who all have the same students on their roles. This allows this group of teachers to get well acquainted with each of the students over the course of the year and share information that may benefit everyone. Others will have a different teacher for every subject, like high school, and their will be no structured relationship at all amongst the teachers as far as having the same students. This can be a huge transition for 11 or 12 year olds either on the cusp of or in the throws of adolescence.
PBS Kids has a great website to help elementary students who are on their way up or middle school students who have recently landed to explore their own thoughts and feelings about moving to the next level as well as gain valuable information and ideas. It includes a journal page, a list of recommended books, as well as questions they can talk about with their parents or adults at their school. This is great information for elementary or middle school counselors to use either with all of their students or a targeted group or individuals that seem to really be struggling with the switch. What I like about it is that it is coming at it from a strengths-based perspective--what does the individual child bring with them that will help them be successful? What aspects of the middle school we the child be drawn to and see as positive?
It can be a scary transition, but helping them to get answers to their questions ahead of time, process their feelings, and find things about the new school that they will enjoy will help to make it a little easier.