If you are at a loss as to what new middle school students struggle with, contact the middle school teachers! I used this strategy, too, just to be sure I was on target. With the earlier enrollment dates, it also became apparent that a follow-up session on transitions would be beneficial. A few years back, the school where I was had a few former students come and answer questions that the fifth grade students had about middle school. It was incredibly successful, because the kids were getting feedback from kids who had been in their shoes the year before.
It's easy to forget that these kids, in many cases, are leaving the only school they have ever attended for something new. It's new students, new experiences, and new environments. Like we would be, our babies are nervous, so when planning your transition lessons, find ways to ease some of their worries.