--方法一：

select a.id,a.SName,a.ClsNo,a.Score

    from Table1 a left join Table1 b on a.ClsNo=b.ClsNo and a.Score<b.Score

        group by a.id,a.SName,a.ClsNo,a.Score

        having count(b.id)<2

    order by a.ClsNo,a.Score desc

--方法二：

select *

    from Table1 a

        where 2>(select count(*) from Table1 where ClsNo=a.ClsNo  and Score>a.Score)

    order by a.ClsNo,a.Score desc

--方法三：

select *

    from Table1 a

        where id in (select id from Table1 where ClsNo=a.ClsNo order by Score desc limit 2)

    order by a.ClsNo,a.Score desc