Both parents and middle schools have important roles to play to ease teens' transition into high school. Ideally, your teen's middle school has a formal transition program in place, which you should definitely participate in with your teen. If not, the suggestions below can help you advocate for such a program or develop one for your teen on your own.
High schools are often much larger than middle schools and provide teens with many more choices. The more your teens know about their options -- how the high school operates and what to expect before high school begins -- the better your teen will manage the change. To get your teen exposed to the high school before he/she is actually a student there, you and your teen can attend some public high school activities, tour the school, meet with current students and teachers, and review class choice options. You and your teen might also meet with one of the high school counselors to discuss the school and your teen's concerns specifically. If the high school runs an orientation assembly, definitely attend, as understanding the rules and organization of the school will help your teen acclimate to the high school's expectations of his/her behavior.
You can also help your teen develop new skills that will help him/her be successful in high school. For example, high school demands greater time management and studying strategies than middle school. Help your teen develop these skills prior to the beginning of the new school year. Your teen's social network will get disrupted with the transition into high school. Prepare your teen for this by getting him/her involved in new activities that give him/her the chance to meet some of the high school students -- ideally, prior to the school year starting. Lastly, transitioning to high school will be stressful, even with the best of planning. So make sure your teen knows you are there for him/her to talk to about fears and challenges, and to support him/her in this new environment.