The proposed tactic
<xsl:choose>
<xsl:when test="scores/scores.low_net_score = 1">
<tr class="something">
</xsl:when>
<xsl:otherwise>
<tr class="otherthing">
</xsl:otherwise>
</xsl:choose>
doesn't work because XSLT wants proper nesting of elements. If you
think about the output being a tree structure rather than a text
stream, the reasons may become clearer.
But you can do this:
<tr>
<xsl:attribute name="class">
<xsl:choose>
<xsl:when test="scores/scores.low_net_score=1">something</xsl:when>
<xsl:otherwise>otherthing</xsl:otherwise>
</xsl:choose>
</xsl:attribute>
<!-- The rest of the stuff that goes in this tr -->
</tr>
For further guidance, get one of the many books about XSLT.
.................David Marston