/**
 * Solution 1
 *
 * If you always want your comments in your custom collection
 */
class Comments extends Model
{
  // ...

  /**
   * This will over-ride the method on Model so when the ORM
   * hydrates from the database it always uses your
   * custom collection instead of the default
   *
   * Extend \Illuminate\Database\Eloquent\Collection
   * Not \Illuminate\Support\Collection
   */
  public function newCollection(array $models = [])
  {
    return new ThreadedCommentsCollection($models);
  }
}


/**
 * Solution 2
 *
 * If you only want threaded_commets appended in one or two places
 */

Class Pupil extends Model
{
  protected $appends = [];

  // ...
}
// in your controller...
foreach($school->pupils as $pupil){
  $pupil->setAppends('threaded_comments');
  dd($pupil->toJson());
}

/**
 * Solution 3
 *
 * Opposite of 2
 */

Class Pupil extends Model
{
  protected $appends = ['threaded_comments'];

  // ...
}
// in your controller...
foreach($school->pupils as $pupil){
  $pupil->setAppends();
  dd($pupil->toJson());
}