When I was at school, the school would take their own photos during the dress rehearsal. They'd make sure there was a whole class one, a whole performance (if more than one class), one of each "group" eg shepherds, narrators, oddments left over... and lots taken during a dress rehearsal.
They charged 10p per photo to order.
I do remember once where they took all the photos from one angle of the ones during the rehearsal. I was on stage the whole time. Every photo had me in... well one bunch of hair sticking out one side of the pillar, and a knee and other bunch sticking out the other side. 
However I would like to go back to that. In these days it could be even easier. Someone comes and takes lots of snaps of everything and puts them on a CD. You can sell that for £2 and make a profit. And then ban photos/cameras for the main show.
Or do a "photo show" where people can come and take photos, but not allow them for the main one.
It seems that too many parents, often those who are otherwise sensible, think that taking a photo of their child at that particular moment, takes priority over the people behind seeing their child, possibly doing their one line.