Código PHP:
<?
include ("../conexion.php");
//Ejecutamos la sentencia SQL
$result=mysql_db_query($db,"select * from plantel ORDER BY numero ASC") or die (mysql_error());
$rows =mysql_fetch_array($result);
for ($i=1; $i<=45; $i++){
if(!in_array($i, $rows)){
echo '<option value="'.$i.'">'.$i.'</option>';
}
}
mysql_free_result($result);
?>