If you give a trial lesson then there must be a qualified instructor giving the lesson.
If the trial flight does not involve the passenger manipulating the controls ie a pleasure flight, then the flying school would need to have an AOC as this would be a pleasure trip. As such if the pilot was being paid for doing the trip, even if his pay was as a desk clerk then he would need to hold the appropriate licence. That would be a CPL.
The only way a PPL can take passengers for pleasure is if he is paying for a proportion of the flight. If he has two passengers then he must pay one third of the price of the trip to stay legal.
If the school has no instructors available and no AOC then the aircraft should stay on the ground.
If I'm wrong then I've just wasted a shed load of money over the last three years getting a CPL/IR and instructor rating.
Please let me know!