Custom User Roles Not Working Correctly

So I have removed all user types using

add_role('student', 'Student', array(
    'read' => true,
    'edit_posts' => false,
    'delete_posts' => false,
));

add_role('teacher', 'Teacher', array(
    'read' => true,
    'edit_posts' => false,
    'delete_posts' => false,
));

remove_role('editor');
remove_role('author');
remove_role('contributor');
remove_role('subscriber');

and set the default user to be a student. But when I create a new user firstly it doesn’t actually default to being a student. and when I look in the database it still has
a:2:{s:10:"subscriber";b:1;s:7:"student";b:1;}

so I added a hook to try change this

add_action('user_register','user_signup_mod_default',10,1);
function user_signup_mod_default($user_id){
        $user = new WP_User($user_id);
        $user->remove_role('subscriber');
        $user->set_role('student');
}

But this has also not worked. So what I really want to know is why does the subscriber role still come up in the database? Is this some kind of default ? Any help be great.

Thanks