Using WP_List_Table to Create WordPress Admin Tables

I have tried this on my own code, where I use custom post type and custom database table. I have changed the code a little, but still get the same error. This is the code for the dele:

function column_student($item){
        $actions=array(
        'edit'=>sprintf('<a href="?post_type=%s&page=%s&action=%s&student=%s">Edit</a>',$_REQUEST['post_type'], $_REQUEST['page'], 'edit', $item->students_id),
        'delete'=>sprintf('<a href="?post_type=%s&page=%s&action=%s&student=%s">Delete</a>', $_REQUEST['post_type'], $_REQUEST['page'], 'delete', $item->students_id),
        
        );
        //Return the title contents
        return sprintf('%1$s <span style="color:silver"></span>%2$s',
            /*$1%s*/ $item->students_name,
            /*$2%s*/ $this->row_actions($actions)
        );
    }

function process_bulk_action(){
        if( 'delete'===$this->current_action() ) {
                $this->bsp_delete_student(($_REQUEST['student']));
        }
    }
    function bsp_delete_student($id){
        global $wpdb;
        $tablename=$wpdb->prefix."students"; //geting our table name with prefix
        $result=$wpdb->query("DELETE FROM '.$tablename.' WHERE students_id='.$id'");
    }
    
    function column_cb($item){
        return sprintf('<input type="checkbox" name="%1$s" value="%2$s" />', 
        $this->_args['singular'],  //$1%s   we use the tables singular label
        $item->students_id        //$2%s    the value of the chechbox is the students id
        );
    }

The rollover edit and delete are not showing anymore and I don’t know why. But when I click on the apply for bulk action it just redirects me to You do not have sufficient permissions to access this page. In the URL I now see the correct id’s, but nothing happens.
It’s also interesting that when I use your code and just change the database and it’s fields to mine, the mouse over delete “button” is not showing. Is this because I’m working on localhost? And when I try to delete the selected items it gives my an Warning: Cannot modify header information - headers already sent by (…wp-includes\pluggable.php on line 1196, but when I click again on the menu the record is deleted. Any ideas?