The first difference is homework. In High School most teachers will assign you to turn in your homework late if you give them an excuse such as "I forgot it at home" or "It is in my locker". In contrast, in College if you give your professor one of these excuses, they will say "Too bad" and you do not have another opportunuty.
Another difference are the tests. There are also a lot of tests in High School; usually, there is a test every week or two. Althogh in College there are more tests, the work is not hard. Most of the time you will read aloud in class or go over every point of the homework so, there is no question of what you have to do.