This might be the ultimate in flexibility:

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

But it also might be a pain to process.