Mohon Banget Bantuan Dari Para Master PHP

Kenapa Setiap Ane Klik Submit Di Addres bar cuma Keluar "http://localhost/siakad.smpn24seluma/menu_ops.php?kode_kelas=K001&nama_kelas=Tujuh&kode_guru=g002&submit=SIMPAN"

<!-- ini untuk tampil kelas-->
<form action="" method="post" target="_self">
<center><table width="100%" border="6" cellpadding="2" cellspacing="0">
<tr>
<th width="30">Kode Kelas</th>
<th width="30">Nama Kelas</th>
<th width="30">Kode Guru</th>
<th colspan="2" width="50">Tools</th>
</tr>
<?php
include_once ('connections/connectdb.php');
$query = mysql_query("SELECT * FROM data_kelas");

while ($data = mysql_fetch_array($query)){
?>
<tr>
<td width="30"> <?php echo $data['kode_kelas']?></td>
<td width="30"> <?php echo $data['nama_kelas']?> </td>
<td width="30"> <?php echo $data['kode_guru'] ?></td>

<td width="30" align="center"><a href="?page=edit_kelas&kode=<?php echo $data['kode_kelas'];?>">Edit</a></td>
<td width="30" align="center"><a>Hapus</a></td>
</tr>

<?php } ?>
</table></center>



<!-- ini untuk edit kelas-->
<?php
include_once ('connections/connectdb.php');
$kode=@$_POST['kode_kelas'];
$query = mysql_query("SELECT * FROM data_kelas WHERE kode_kelas='$_GET[kode]'")or die (mysql_error());
$data = mysql_fetch_array($query);
?>

<form action="" method="_POST" target="_self"><center><table width="700 px" border="0" cellpadding="3" cellspacing="9">
<tr>
<td>Kode Kelas</td>
<td>:</td>
<td><input type="text"name= "kode_kelas" value="<?php echo $data ['kode_kelas']?>"></td>
</tr>
<tr>
<td>Nama kelas</td>
<td>:</td>
<td><input type="text" name= "nama_kelas" value="<?php echo $data ['nama_kelas']?>" ></td>
</tr>
<tr>
<td>Kode Guru</td>
<td>:</td>
<td><input type="text"name= "kode_guru" value="<?php echo $data ['kode_guru']?>"></td>
</tr>


</table>
<input type="submit" name="submit" value="SIMPAN">
<input type="reset" name="reset" value="BATAL">

</form>
</div>


<center>
</center>
<?php

@$kode_kelas=$_POST['kode_kelas'];
@$nama_kelas=$_POST['nama_kelas'];
@$kode_guru=$_POST['kode_guru'];
if (@$_POST['submit']){$insert= "UPDATE data_kelas SET WHERE kode_kelas='$kode_kelas', nama_kelas='$nama_kelas', kode_guru='$kode_guru'";
@$simpan = mysql_query ($insert,$connectdb)or die ("Gagal".mysql_error()); header ("location:?page=data_kelas");}
?>