<p>It is my impression that at research universities (both private and flagship) , students are generally given the option of coming up to campus at some point during the summer to meet with an advisor and register for classes. (Sometimes placement tests are also involved.) There is often a concern that if students choose to wait until they show up for fall semester, the desired sections will already be full. Nowadays, students can access Rate My Professor or ask about various options on sites such as this one ahead of time, so the student who just shows up to register before classes begin could be at a relative disadvantage.</p>