Each of these elements could have a super type:

<t:student id="s1">
<name>Jimmy</name>
</t:student>

<t:parent id="p1">
<name>Jimmy's Mom</name>
<address>
<street>123 Mars</street>
<city>San Francisco</city><state>CA</state>
</address>
<children>
<student ref="s1"/>
</children>
</t:parent>

<t:teacher>
<name>Dr. Evil</name>
<students>
<student ref="s1"/>
</students>
</t:teacher>

These could all have a base type of:

<xs:complexType name="PersonInfo">
   <xs:sequence>
      <xs:element name="name" type="xs:string"/>
   </xs:sequence>
   <xs:attribute name="id" type="xs:ID"/>
</xs:complexType>