To back up what the other chaps have said, the best ATPL ground instructors are the NON-pilots.
Engineers do the best job of teaching engineering subjects, navigators do the best job with the navigational subjects and for the odd "piloty bits" such as ops procedures and air law, those apply equally to ALL aircrew, whether pilot, nav or flight eng.
Just for interest, suedee, have you managed to find a ground school that IS taught by pilots? I don't know of any!