I believe this will give you what you want;
Names Default To Here( 1 );
dt = Open( "$SAMPLE_DATA/big class.jmp" );
dt << select where(
:HEIGHT < Col Quantile( :Height, .05 ) |
:HEIGHT > Col Quantile( :Height, .95 )
);
dt << exclude;
Or if you need to have your above and below row number matrices, then this will work
Names Default To Here( 1 );
dt = Open( "$SAMPLE_DATA/big class.jmp" );
RowsBelow5PCTL = dt << get rows where( :HEIGHT < Col Quantile( :HEIGHT, .05 ) );
dt << select rows(RowsBelow5PCTL);
RowsAbove95PCTL = dt << get rows where( :HEIGHT > Col Quantile( :HEIGHT, .95 ) );
dt << select rows(RowsAbove95PCTL);
dt << exclude;
Jim