I think you've hit a very touchy subject!
My daughters school have never mentioned anything about filming or photography at her school plays / sports days etc but our local swimming pool won't allow photography.
I'm not sure there is a law as such, I think it's more a case of 'just in case', so they don't get any come back or complaints about people filming etc.
I'm going to my baby girls school play this coming Wednesday, so fingers crossed I can film her, as her school plays are so funny. There is always a twist to them!
As for the Christopher Graham thing, I'd happily show anyone my video's of my little girl as I'm happy to say all I care about is my girl. End of the day, she is fully dressed so can't see any problems.