Sort columns on meta value

Hi
I have a column in the user admin table that i would like to sort by the meta values in the columns.
Currently my code contains (thank you david sader)

add_filter('manage_users-network_sortable_columns', 'ds_column_sortable');
function ds_column_sortable($columns) {
           $columns['rpr_school_year'] = 'id';
	return $columns;
}

however it does not sort by the values in the column..
help please!
TIA