I don't think you have the legal right to interfere with someone else's property in that manner. You can ask them to do so, however if you damage the equipment on which it is taken you may find yourself with a hefty repair bill.
Also how would you delete pictures on film?
If you take a photo in the theatre, the photographs become the property of the venue, legally.
There is a disclaimer on the back of the tickets, on the website and next to the box office that photography is banned during the performance and and equipment used to do so may be confiscated and destroyed:
The use of equipment for recording or transmitting (by digital or other means) any audio, visual or audio-visual material or any information or data inside any Venue is strictly forbidden. Unauthorised recordings, tapes, films or similar items may be confiscated and destroyed. Any recording made of an Event in breach of these conditions shall belong to the Promoter. The Promoter and Venue will not be liable for any loss, theft or damage to confiscated items.
Mobile telephones and messaging equipment must be switched off during the Event.
So film would be removed from the camera and probably destroyed. By buying a ticket, you have agreed to their terms and conditions, so it's all legal and above board.
I'm not saying I'm in agreement with it necessarily, and frankly it was one of the worst parts of the job, but there you have it. Read the T&Cs before you take a picture in a theatre is my advice.
In a school play, this is especially ridiculous though. The whole point of schools plays is so parents can record it and show it to your first boyfriend, years later, surely?