comecei a trabalhar com a ajuda deste post no final. Felicidades pessoal;
$finalArgs = array (
'posts_per_page'=>5,
'order' => 'ASC',
'post_type' => 'school'
);
// Create a new instance
$searchSchools = new WP_Query( $finalArgs );
$mypostids = $wpdb->get_col("select ID from $wpdb->posts where post_title LIKE '".$str."%' ");
$args = array(
'post__in'=> $mypostids,
'post_type'=>'school',
'orderby'=>'title',
'order'=>'asc'
);
$res = new WP_Query($args);
while( $res->have_posts() ) : $res->the_post();
global $post;
$EstablishmentNumber = get_post_meta($post->ID,'EstablishmentNumber', true);
$schl = array('id'=>$EstablishmentNumber, 'label'=>$post->post_title , 'value'=>$EstablishmentNumber );
$matchedSchools[] = $schl;
endwhile;