Get value once that occurs > once but siblings multiple times

Question

The XSLT needs to output something like:

Mathematics
  ID
  ID
  ID
PSE
  ID

I currently get:

MathematicsID
  ID
  ID
PSEID

It seems that when it gets the <Subject> it also gets all the other elements, in this case ID where i want it to get the <Subject> then collect the relevant ID'.

Here is my XML:

<ArrayOfSchoolClass>
  <SchoolClass>
    <ExtensionData />
    <Subject>Mathematics</Subject>
    <ID>9</ID>
  </SchoolClass>
  <SchoolClass>
    <ExtensionData />
    <Subject>PSE</Subject>
    <ID>9</ID>
  </SchoolClass>
  <SchoolClass>
    <ExtensionData />
    <Subject>Mathematics</Subject>
    <ID>9</ID>
  </SchoolClass>
  <SchoolClass>
    <ExtensionData />
    <Subject>Mathematics</Subject>
    <ID>9</ID>
  </SchoolClass>
</ArrayOfSchoolClass>

And the XLST:

 <xsl:key name="SchoolClass" match="SchoolClass" use="Subject"/>
  <xsl:template match="ArrayOfSchoolClass">
   <ArrayOfSchoolClass>
    <xsl:for-each select="SchoolClass[generate-id()=generate-id(key('SchoolClass',Subject)[1])]">
     <SchoolClass>
     <xsl:copy-of select="Subject"/>
     <xsl:for-each select="key('SchoolClass',Subject)">
     <xsl:copy-of select="*[not(self::Subject)]"/>
     <br />
     </xsl:for-each>
     </SchoolClass>
    </xsl:for-each>
   </ArrayOfSchoolClass>
 </xsl:template>

Im really trying to create a menu where <Subject> is the Heading and <ID> is the link underneath without getting multiple <Subject> headings that arent unique, if I can just work out how to select the elements correctly ....

Something like:

<ul>Mathematics
  <li>9</li>
  <li>9</li>
</ul>
<ul>PSE</ul>
  <li>9</li>
</ul>