PHP MySQL DELETE row from html table with link button

I've been looking at this for some time now, all I'm doing is learning php with mysql, with no previous programming experience before these so please be kind. I need to delete an entire row from a table. The table is inside list.php and there's a delete button that redirects to delete.php. But all it does it redirects me to a blank page with an url something like "delete.php?id=4" depending on row ID, which seems to be correct.

This is connect.php:

<?php 
$server = "localhost";
$db_user = "root";
$db_pass = "";
$db_name = "school";

$db_conn = mysqli_connect($server, $db_user, $db_pass, $db_name);

$connect_error = 'Sorry, we are experiencing connection problems';

mysql_connect('localhost', 'root', '') or die($connect_error);
mysql_select_db('school') or die($connect_error);

?>

And this is delete.php :

<?php 
if(isset($_GET['ID'])){
    $courseID = $_GET['ID'];
    $sql_delete = "DELETE FROM Courses WHERE ID = $courseID";       
    print ($sql_delete);        
        $result = mysqli_query($db_conn,$sql_delete);
    if($result) {
        echo "Congratulations. You have deleted this course succesfully.";
        header('location:list.php');
    } else {
        echo "Error";
    error_reporting(E_ALL ^ E_DEPRECATED);
    }

}
?>

Notice "print ($sql_delete);", which doesn't print anything either. All I can think of is there is something wrong with the way I am asking for the ID by using $_GET, but can't get my head around it. When I run

"DELETE FROM Courses WHERE ID = 4;"

inside xampp's mysql module, it works. Uhm... oh well.

OK, let me add: This is actually my delete button inside list.php, which gives me an URL based on table row number, based on Course id.

<?php echo "<td><a href=\"delete.php?id=$row[ID]\">Delete</a>";?>