After my first year of high school I have notice some very large differences between high school and middle school. Please do understand that I have also changed schools. I have been able to narrow the major differences into three different groups: physical, mental, and social.
The physical differences are the most obvious. The first is the weight of my backpack. On average it weighs anywhere from 25 to 65 pounds each night. This of course can change easily depending on the classes you take and how effective you use your study hall. The next difference is that school is an open campus. Students can move around the building a lot going from class to class and floor-to-floor. This can be a problem if you have friends like mine who see you in class and try to communicate with you, mainly by trying to make you laugh in class. Another physical difference is that the bus is one of the best places to study. Even though the ride is very bumpy if you have a good short-term memory you can easily ace a test by cramming during the test. In the long run you may not remember the work, but at least you ace your test.
There is also a heavy mental difference in high school. There is pressure to succeed in high school. This can come from a rivalry with friends or anyone or pressure from your parents. If it is a rivalry with friends it can be very fun when you are winning. If it is the latter of the two if you mess up you it can end up poorly for you. Also your classes are harder or are supposed to be harder. There is also (at least for me) a creeping in of what you want to be or what collage you want to get into.
And finally there is the social aspect. You will notice that your friend group expands and evolves. This is especially true if you are coming from a new school. You are also introduced to many new ideas. With these new ideas you may like them and you may not, it is just part of life.