<root>
    <discipline>
        <row DisciplineID="8562" Name="Информатика" />
    </discipline>
    <students>
        <row StudentID="2830" LastName="Пупкин" SumResult="90"/>
    </students>
    <balls>
        <row StudentID="2830" DisciplineID="8562" Result="90" />
    </balls>
</root>

          <tbody>
            <xsl:variable name="unsorted">
              <xsl:apply-templates select="STUDENTS/*" mode="tosort"/>
            </xsl:variable>
            <xsl:for-each select="msxsl:node-set($unsorted)/*">
              <xsl:sort select="sum" order="descending"/>
              <xsl:sort select="r1" order="descending"/>
              <xsl:sort select="r2" order="descending"/>
              <xsl:sort select="r3" order="descending"/>

              <xsl:variable name="id" select="id"/>

              <!-- это выводится -->
              <xsl:value-of select="$id"/>.

              <!-- а это нет -->
              <xsl:apply-templates select="//STUDENTS/*[@OrderGoodsStudentID = $id]" mode="tr"/>
            </xsl:for-each>
          </tbody>

      <xsl:template match="STUDENTS/*" mode="tosort">
        <xsl:variable name="ogs" select="@OrderGoodsStudentID"/>
        <student>
          <id><xsl:value-of select="@OrderGoodsStudentID"/></id>
          <sum><xsl:value-of select="@SumResult"/></sum>
          <xsl:for-each select="/UWS/GOODS/*">
            <xsl:element name="r{position()}">
              <xsl:value-of select="/UWS/PRELIMINARY/*[@OrderGoodsStudentID = $ogs][@GoodsID = current()/@GoodsID]/@Result"/>
            </xsl:element>
          </xsl:for-each>
        </student>
      </xsl:template>

      <xsl:template match="STUDENTS/*" mode="tr">
        <xsl:variable name="ogs" select="@OrderGoodsStudentID"/>
        <tr>
          <td><xsl:value-of select="position()"/></td>
          <td><xsl:value-of select="@LastName_RU"/></td>
          <xsl:for-each select="/UWS/GOODS/*">
            <td><xsl:value-of select="/UWS/PRELIMINARY/*[@OrderGoodsStudentID = $ogs][@GoodsID = current()/@GoodsID]/@Result"/></td>
          </xsl:for-each>
          <td><xsl:value-of select="@SumResult"/></td>
        </tr>
      </xsl:template>