<?php
		   include ('blocks/bd.php');
        $result = mysql_query('SELECT id_class  FROM users ');
				 $myrow = mysql_fetch_array($result);
				 
				 do
				 	 {
					 printf("<p align='center'><option id='single'>%s</option>",$myrow["id_class"]);
					 }
				 while ($myrow = mysql_fetch_array($result));
        ?>

mysql_query('SELECT `id_class` FROM `users` GROUP BY `id_class`;');