Filters

add_filter( 'rtm_modify_rating_label', 'rtmedia_modify_rating_label', 10, 1 );

function rtmedia_modify_rating_label ( $labels ) {
    $labels['rating'] = 'Grade';
    $labels['your_rating'] = 'Your Grade';
    return $labels;
}

add_filter( 'rtmedia_render_media_rate', 'rtm_render_media_rating', 10, 1 );

function rtm_render_media_rating ( $flag ) {
    return true;
}