Maybe 'student' should be a child of university...

<university>
 <name>UC Berkeley</name>
 <student id="s1"><name>Jane Smith</name></student>
 <student id="s2"><name>John Doe</name></student>
 <department>
  <name>SIMS</name>
  <class id="290-4">
    <student ref="s1"/>
    <student ref="s2"/>
  </class>
  <class id="290-8">
    <student ref="s2"/>
  </class>
 </department>
</university>

But departments can cross-list courses and students can be from other universities...