$turmas= $wpdb->get_results($wpdb->prepare("SELECT nome FROM turmas WHERE id= '%d' ", $_POST['idSerie']),ARRAY_A) ;

$count = count($_POST['idSerie']);
$turmas[];
for ($i = 0; $i < $count; $i++)
    {
        $turmas[] = $wpdb->get_results($wpdb->prepare("SELECT nome FROM turmas WHERE id= '%d' ", $_POST['idSerie'][i]),ARRAY_A) ;
    }