Código PHP:
include ("config/index.php");
$query = "SELECT * from usuarios where centro='$mi_cole'";
$result = mysql_db_query ($dbname, $query, $link);
$num_fila = 0;
$posicion = 1;
while ($row = mysql_fetch_array ($result)) {
$alumno = $row['nick'];
//Calculo la media
$query_media_alumno = mysql_query("select AVG(nota) from examenes where autor='$alumno'");
$media_alumno_x = mysql_result($query_media_alumno,0);
$media_alumno = round ($media_alumno_x,1);
//muestro la lista.......................................
$posicion++;
}