<p>ycang, certain classes are attended by freshman only, such as the first writing class. But most classes have a mix of grades. You will find freshmen taking a 300 upper division level class in their major, and conversely upperclassmen taking a 200 level class in their major or a particular GE or language class.
If you have AP credits, sometimes you can be exempt from an intro class and take the next level, and sometimes you need D clearance (Departmental) to take that class, which your adviser will guide you through at orientation/registration.</p>