class Users_Model extends CI_Model{

public function __construct(){
    parent::__construct();
}

class Students_Model extends Users_Model{

private $_students;

public function __construct(){
      parent::__construct();

      $this->_students = $this->get_students();
    }

PHP Fatal error:  Class 'Users_Model' not found in /Users/rosswilson/Localhost/thedrumrooms/dreamweaver/ci/application/models/students_model.php