Well, there are many differences between high school and university. I studied in Claret´s school for 12 years and I can say there´s nothing as my school. I made my best and true friends there.
In high school you don´t have many responsibilities, you´re only thinking about having fun and spend a good time with your close friends. You´re expecting the graduation and the travel you´re gonna make after it.
On the other hand, once you get in to the university, you can see the differences from the beginning. In the university you are the only responsible of your future, and all the decisions that you take are going to influence in it. Also, it´s not so easy making new friends, because everybody is in their own world. Subjects are harder and you have to being constantly studying.