例えば
<SCHOOL>
    <STUDENT NAME="BOB">
        <GRADE SCORE="50"/>
        <GRADE SCORE="10"/>
        <GRADE SCORE="20"/>
        <GRADE SCORE="40"/>
        <GRADE SCORE="90"/>
        <GRADE SCORE="30"/>
    </STUDENT>
</SCHOOL>
に変換
<SCHOOL>
    <STUDENT NAME="BOB">
        <GRADE SCORE="50"/>
        <GRADE SCORE="10"/>
        <GRADE SCORE="20"/>
    </STUDENT>
    <STUDENT NAME="BOB">
        <GRADE SCORE="40"/>
        <GRADE SCORE="90"/>
        <GRADE SCORE="30"/>
    </STUDENT>
</SCHOOL>
xslt を使い始めたばかりで、これを達成する方法がわかりません!