undefind index issues

[knewts98]

am new in php and what to insert data in my database from a form but when i try inserting i get only half of my data in the database. here is my code.
Please help me out. Thanks in advance.

<?php include 'muthala primary.php';?>
 
<?php
 
 
// create a variable
$Student_ID  = isset($_POST['Student_ID']) ? $_POST['Student_ID'] : '';
$first_name  = isset($_POST['first_name']) ? $_POST['first_name'] : '';
$last_name  = isset($_POST['last_name']) ? $_POST['last_name'] : '';
$Class_name  = isset($_POST['Class_name']) ? $_POST['Class_name'] : '';
$Gender  = isset($_POST['Gender']) ? $_POST['Gender'] : '';

$Date_of_Birth  = isset($_POST['Date_of_Birth']) ? $_POST['Date_of_Birth'] : '';

$Index_Number  = isset($_POST['Index_Number']) ? $_POST['Index_Number'] : '';

 
//Execute the query
 
 mysqli_query($connect,"INSERT INTO students (Student_ID, first_name, last_name, Class_name, Gender, Date_of_Birth, Index_Number) VALUES ('$Student_ID', '$first_name', '$last_name', '$Class_name', '$Gender', '$Date_of_Birth', '$Index_Number')");
			
if(mysqli_affected_rows($connect) > 0){
	echo "<p>student Added</p>";
	echo "<a href= \"student.php\">Go Back</a>";
} else {
	echo "student NOT Added<br />";
	echo mysqli_error ($connect);
}
?>