wp_list_table (not working)

gistfile1.php

//Our class extends the WP_List_Table class, so we need to make sure that it's there
if(!class_exists('WP_List_Table')){
   require_once(ABSPATH .'wp-admin/includes/class-wp-list-table.php');
}

//The WP List table class
class bsp_list_table extends WP_List_Table {

	 /**
    * Constructor, we override the parent to pass our own arguments
    * We usually focus on three parameters: singular and plural labels, as well as whether the class supports AJAX.
    */
    
    function __construct(){
		
   		parent:: __construct(array(
		    'singular'=>"student",
		    'plural'=>"students",
	    	'ajax'=>false
		));
	}

	public function prepare_items(){
		
		global $wpdb, $_wp_column_headers;
	//	$screen=get_current_screen();
		$tablename=$wpdb->prefix."students"; //geting our table name with prefix
		
		//selecting every thing from table
		$data="SELECT * FROM $tablename";
		
		$this->process_bulk_action();
		
		//if no order, default to asc
		$orderby = !empty($_GET['orderby']) ? mysql_real_escape_string($_GET['orderby']) : 'ASC';
		//if no order leave it blank (as it is) - we can order it by name or id or...
        $order = !empty($_GET['order']) ? mysql_real_escape_string($_GET['order']) : '';
        //if it's not empty then query it from databse
        if(!empty($orderby) & !empty($order)){ $data.=' ORDER BY '.$orderby.' '.$order; }
		 /* -- Pagination parameters -- */
        //Number of elements in your table?
        $totalitems = $wpdb->query($data); //return the total number of affected rows
        //How many to display per page?
        $perpage = 5;
        //Which page is this?
        $paged = !empty($_GET['paged']) ? mysql_real_escape_string($_GET['paged']) : '';
        //Page Number
        if(empty($paged) || !is_numeric($paged) || $paged<=0 ){ $paged=1; }
        //How many pages do we have in total?
        $totalpages = ceil($totalitems/$perpage);
        //adjust the query to take pagination into account
        if(!empty($paged) && !empty($perpage)){
            $offset=($paged-1)*$perpage;
            $data.=' LIMIT '.(int)$offset.','.(int)$perpage;
        }
        /* -- Register the pagination -- */
        $this->set_pagination_args( array(
            "total_items" => $totalitems,
            "total_pages" => $totalpages,
            "per_page" => $perpage,
        ) );
        //register the columns
        $columns  = $this->get_columns();
        $hidden   = array();
        $sortable = $this->get_sortable_columns();
        $this->_column_headers = array( $columns, $hidden, $sortable );
        
        //fetch the items
        $this->items=$wpdb->get_results($data,'ARRAY_A');
        
 
	}
	
	function get_columns(){
		$columns=array(
		'cb'=>'<input type="checkbox" />',
		'students_name'=>__('Name'),
		'students_lastname'=>__('Lastname'),
		'students_email'=>__('E-mail')
		);
		
		return $columns;
	}
	
	
	function get_sortable_columns(){
		$sortable=array(
		'students_name'=>array('students_name', false),
		'students_lastname'=>array('students_lastname', false)
		);
		return $sortable;
	}
	
	
	function column_default($item, $column_name){
		switch($column_name){
			//case 'students_id':
			case 'students_name':
			case 'students_lastname':
			case 'students_email':
			return $item[$column_name];
			
			default:
			//for debugging purposes we print out the whole array
			return print_r($item, true);
		}
	}
	
	function column_name($item){
	
		$delete_nonce = wp_create_nonce( 'bsp_delete_student' );
		$title = '<strong>' . $item['students_name'] . '</strong>';
		$actions=array(
		
		'delete' => sprintf( '<a href="?post_type%s&page=%s&action=%s&student=%s&_wpnonce=%s">Delete</a>', esc_attr( $_REQUEST['page'] ), $_REQUEST['post_type'], 'delete', absint( $item['students_id'] ) )
 
		//'delete' => sprintf( '<a href="?page=%s&action=%s&student=%s&_wpnonce=%s">Delete</a>', esc_attr( $_REQUEST['page'] ), 'delete', absint( $item['students_id'] ), $delete_nonce )
		
		);
		
		return $title . $this->row_actions( $actions );
		/*//Return the title contents
        return sprintf('%1$s <span style="color:silver">(id:%2$s)</span>%3$s',
            /*$1%s/ $item['students_name'],
            /*$2%s/ $item['students_id'],
            /*$3%s/ $this->row_actions($actions)
        );*/
		
	}
	
	function get_bulk_actions(){
		$actions=array(
		'delete'=> __( 'Delete' )
		);
		return $actions;
	}	
	
	public function process_bulk_action() {

		//Detect when a bulk action is being triggered
		if ('delete' === $this->current_action() ) {
			 
			// In our file that handles the request, verify the nonce.
			$nonce = esc_attr( $_REQUEST['_wpnonce'] );

			if ( !wp_verify_nonce( $nonce, 'bsp_delete_student' ) ) {
				die( 'Go get a life script kiddies' );
			}
			else {
				self::delete_student( absint( $_GET['student'] ) );

				wp_redirect( esc_url( add_query_arg() ) );
				exit;
			}

	}

		// If the delete bulk action is triggered
		if ( ( isset( $_POST['action'] ) && $_POST['action'] == 'delete' )
		     || ( isset( $_POST['action2'] ) && $_POST['action2'] == 'delete' )
		) {

			$delete_ids = esc_sql( $_POST['delete'] );

			// loop over the array of record IDs and delete them
			foreach ($delete_ids as $id ) {
				self::delete_student( $id );

			}

			wp_redirect( esc_url( add_query_arg() ) );
			exit;
			//wp_die('Items deleted (or they would be if we had items to delete)!');
		}
	}
	
	//function for deleting a student from the table
	public static function delete_student($id){
		global $wpdb;
		$wpdb->delete("{$wpdb->prefix}students",
			[ 'students_id' => $id ],
			[ '%d' ]
		);
	}
	
	//ading the checkboxes to the table %1$s we use the tables singular label, %2$s the value of the chechbox is the students id
	function column_cb( $item ) {
		return sprintf('<input type="checkbox" name="%1$s[]" value="%2$s" />', 
			$this->_args['singular'], 
			$item['students_id']
		);
	}
	
	function no_items() {
		_e( 'The database is empty.' );
	}
	

}//end of class

//function for displaying the custom table data in wp_list_table
function bsp_students_show(){
		//new instance of wp_list_table
		$bsplisttable = new bsp_list_table();
	//outputing the headline before table
	echo '<div class="wrap"><h2>'. __('Enroled students').'</h2>';
	//calling function prepare_items
		$bsplisttable->prepare_items();
	?>
	<!-- Forms are NOT created automatically, so you need to wrap the table in one to use features like bulk actions -->
    <form method="post">
        <!-- For plugins, we also need to ensure that the form posts back to our current page -->
        <input type="hidden" name="page" value="<?php echo $_REQUEST['page'] ?>" />
        <!-- Now we can render the completed list table -->
        <?php
        
        $bsplisttable->display();
        ?>
    </form></div>
    <?php
	
}//end of function