Sorting posts by Multiple custom fields in defined order

I'm currently sorting some posts out by custom fields 'role' and 'surname', using the following:

function customorder($orderby) {
    return 'mt1.meta_value DESC, mt2.meta_value ASC';
}

$args = array(
    'post_type'     => 'staff', 
    'meta_key'      => 'role',
    'meta_query'    => array(
        array(
        'key' => 'role'
        ),
        array(
        'key' => 'surname',
        ),
    )
);

add_filter('posts_orderby','customorder');
$query = new WP_Query( $args );
remove_filter('posts_orderby','customorder');

How can I define the order that I would like the first meta_key in, as opposed to alphabetically desc or ascending. For example where role = 'Head Teacher', 'Deputy', 'Teacher', 'Support', 'Clerical' ... etc Therefor grouping the posts by the meta_key value.

Thanks in advance for any help

Cheers

Noel