สอบถามเรื่อง session ถูกลบ เมื่อ submit form ไปอีกหน้าหนึ่ง

<?php
     $_SESSION['school']=$school;
?>

<form role="form" name="form" method="post" target="_blank">
<button type="button" class="btn btn-primary btn-sm" onclick="PrintPage1(this.form);"><i class="fas fa-print"></i></button>
<button type="button" class="btn btn-success btn-sm" onclick="PrintExcel1(this.form);"><i class="far fa-file-excel"></i></button>
</form>
<script type="text/javascript">
//พิมพ์รายงาน
	function PrintPage1(frm)
	{
		frm.action="printreport.php?print=club_cover_print";
		frm.submit();
	}
	function PrintExcel1(frm)
	{
		frm.action="printexcel.php?print=club_cover_print";
		frm.submit();
	}

	function PrintPage2(frm)
	{
		frm.action="printreport.php?print=club_member_print";
		frm.submit();
	}
	function PrintExcel2(frm)
	{
		frm.action="printexcel.php?print=club_member_print";
		frm.submit();
	}
</script>

<?php
	ob_start();
	session_start();
	require_once("connectdb.php");
	require_once("schoolDB.php");
	require_once("function/function.php");
	require_once("function/thaidate.php");
	$print=isset($_REQUEST['print'])? $_REQUEST['print']:null;

	if(empty($print)){
		$include_file = "error_404p.php";
	}else{
		if(file_exists("report/".$print.".php")){
			$include_file = $print.".php";
		}else{
			$include_file = "error_404p.php";
		}
	};
	include("report/".$include_file);
?>

<?php
	//ดึงข้อมูลโรงเรียนมาพักไว้
		$cfquery=mysql_query("SELECT * FROM config WHERE cfschool_id='".$_SESSION['school']."' ");
		if(mysql_num_rows($cfquery)<>0){
			$fcf=mysql_fetch_array($cfquery);
			$tprefix=$fcf["cftable_prefix"];
		}else{
			echo "<script>window.location='../index.php';</script>";
		}

?>