It's a popular storyline that has been repeated many times. Somebody with a bit of personality comes into a school and revolutionises things. The other teachers are sceptical but all the pupils love them, with all the fun activities and excitement. The public says "We need more people like him in teaching etc." There don't seem to be any exams in this world.
Enter Gareth Malone, teaching the kids outside, throwing flour bombs and going camping. He doesn't seem to need to fill out long risk assessment forms for any of this and is unaffected by health and safety requirements. The kids don't run away, start fighting or fall over and injure themselves. The head never tells him he can't do anything, or calls him in for a coffeeless interview and the parents never try and sue the school (well none of this happens on camera anyway.
My point is this: it's easy-peasy to come in to a school, be completely wacky, get the kids cheering for you and declare the whole thing a success. Then you can clear off, leaving the regular staff to pick up the pieces and answer questions like "Why can't we climb a tree and play with the chainsaw like we did with Mr Chalk?"