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.