Need help in PHP

Hi all, I'm working on a project but the function that I want doesn't work.

There is this button called "add student" and a new row will appear when I clicked that button. However the data I wrote in the previous first row will disappear when I click that button for a 2nd row.

Here is the code

code:
<?php
session_start();
if (isset($_POST['AddStudent'])) {
// If user click on Add new student button
$temp = $_SESSION['numberOfStudents'];
$temp = $temp + 1;
$_SESSION['numberOfStudents'] = $temp;
}
/*else if (isset($_POST['FinalCheckBtn'])) {
// If user click on Final Check button go to Final check page
header('Location: http://localhost/SJAA/final_check.php') ;
}*/
else {
// if User enter this page the first time from "create new class page "
// the variable that hold the credentials to connect to the database
$dbserver = "localhost";
$dbuser = "root";
$dbpassword = "";
$dbname = "fyp - sem 1";
// connect to DB
mysql_connect($dbserver, $dbuser, $dbpassword) or die(mysql_error());
// select the correct db.
mysql_select_db($dbname) or die(mysql_error());

// Read variables from the $_POST array
$class_name = $_POST['class_name'];
$class_venue = $_POST['class_venue'];
$date_of_lesson =$_POST['date_of_lesson'];
$time_of_lesson =$_POST['time_of_lesson'];
$class_duration =$_POST['class_duration'];
$date_of_exam =$_POST['date_of_exam'];
$class_in_charge =$_POST['class_in_charge'];
$contact =$_POST['contact'];
$email =$_POST['email'];

// Prepare the query

$sql = "INSERT INTO b_class (class_name,class_venue,date_of_lesson,time_of_lesson,class_duration,date_of_exam,class_in_charge,contact,email) VALUES
('$class_name','$class_venue','$date_of_lesson','$time_of_lesson','$class_duration','$date_of_exam','$class_in_charge','$contact','$email')";

// Execute the query
mysql_query($sql) or die(mysql_error());
$_SESSION['numberOfStudents'] = 0;
$_SESSION['class_name']=$_POST['class_name'];
$_SESSION['class_venue'] = $_POST['class_venue'];
$_SESSION['date_of_lesson'] = $_POST['date_of_lesson'];
$_SESSION['time_of_lesson'] = $_POST['time_of_lesson'];
$_SESSION['class_duration'] = $_POST['class_duration'];
$_SESSION['date_of_exam'] = $_POST['date_of_exam'];
$_SESSION['class_in_charge'] = $_POST['class_in_charge'];
$_SESSION['contact'] = $_POST['contact'];
$_SESSION['email'] = $_POST['email'];

}
?>
<html>
<form enctype="multipart/form-data" action = 'student_details.php' method='post' />
<img style="width: 190px; height: 184px;" alt="" src="images/Picture1.jpg">

<head><title>Students' Particulars</title></head>
<h1>Students' Particulars</h1>
<h3> Please enter student's particulars. All fields are to be filled. </h3>
<table style="text-align: left; width: 333px; height: 144px;"
border="1" cellpadding="2" cellspacing="2">
<tbody>
<tr>
<td><?php
echo "Class Name: ";
echo $_SESSION['class_name'];?> </td>
</tr>
<tr>
<td><?php
echo "Class Venue: ";
echo $_SESSION['class_venue'];?> </td>
</tr>
<tr>
<td><?php
echo "Date of Lesson: ";
echo $_SESSION['date_of_lesson'];?> </td>
</tr>
<tr>
<td><?php
echo "Time: ";
echo $_SESSION['time_of_lesson'];?> </td>
</tr>
<tr>
<td><?php
echo "Class Duration: ";
echo $_SESSION['class_duration'];?> </td>
</tr>
<tr>
<td><?php
echo "Tentative Date of Exam: ";
echo $_SESSION['date_of_exam'];?> </td>
</tr>
<tr>
<td><?php
echo "Class-In-Charge: ";
echo $_SESSION['class_in_charge'];?> </td>
</tr>
<tr>
<td><?php
echo "Contact: ";
echo $_SESSION['contact'];?> </td>
</tr>
<tr>
<td><?php
echo "Email: ";
echo $_SESSION['email'];?> </td>
</tr>
<body>
<table border="1">
<tr>
<th>Name</th>
<th>NRIC no.</th>
<th>Age</th>
</tr>
<br>
<input type="submit" value="Add new Student" name="AddStudent">
</form>
<?php
$cnt = 0;
echo "<form enctype='multipart/form-data' action = 'final_check.php' method='post'>";
while ($cnt < $_SESSION['numberOfStudents']) {

$NameCnt = "Name".$cnt;
$NRICcnt = "NRICno".$cnt;
$AgeCnt = "Age".$cnt;

if (isset($_POST[$NameCnt])) {
?>
<tr>
<td><input type = 'text' name = 'student_name<?php echo $cnt ?>' value='<?php echo $_POST[$NameCnt] ?>'></td>
<td><input type = 'text' name = 'nric<?php echo $cnt ?>' value='<?php echo $_POST[$NRICcnt] ?>'></td>
<td><input type = 'text' name = 'age<?php echo $cnt ?>' value='<?php echo $_POST[$AgeCnt] ?>'></td>
</tr>
<?php
}
else {
?>
<tr>
<td><input type = 'text' name = 'student_name<?php echo $cnt ?>'></td>
<td><input type = 'text' name = 'nric<?php echo $cnt ?>'></td>
<td><input type = 'text' name = 'age<?php echo $cnt ?>'></td>
</tr>
<?php
}

$cnt = $cnt +1;
}
?>

</table>
<br>
<br>
<tr>
<td></td>
<table style="text-align: left;"
<td><input type = 'submit' value = 'Continue' name='FinalCheckBtn'></td>

</tr>
</table>
</form>
</html>

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