Looping total not same in database

Hello i have a script looping php, and each loop 4 data/row then show ads.

but i have a problem.. because in database i have data 123 row. but in show in php only 92 row/data :'(

this is my script

<?php 
include "connection.php";
$i=0; 
$data_school=mysql_query("select * from school");
while ($school=mysql_fetch_object($data_school)) 
{
    if($i%4==0)
    { 
    echo "<br/><a href='#'><img src='ads.jpg' alt=''></a><br/>";
    }
    else 
    { 
        echo $school->name_school"<br/>";
    }
$i++;
}
?>

when i run in browser, and ads show but school only show "92 row/data"

if i remove script

 if($i%4==0)
        { 
        echo "<br/><a href='#'><img src='ads.jpg' alt=''></a><br/>";
        }

then script be like this

<?php 
    include "connection.php";
    $i=0; 
    $data_school=mysql_query("select * from school");
    while ($school=mysql_fetch_object($data_school)) 
    {
            echo $school->name_school"<br/>";

    $i++;
    }
    ?>

this data show all 123 row/data

help me, i want like this

school 1

school 2

school 3

school 4

#ads

school 5

school 6

school 7

school 8

#ads

Help me, Thank's