holy crap. It was over a year ago. You really have to let it go.
They took your comments into consideration and made a change based on them.
They aren't having massive summits with the head of the LEA, Nicky Morgan and Dean Spielmann because it's not a big deal
They're setting up a hypothetical situation. The children are not harmed in any way and won't turn into habitual liars, saying well, it must be ok because when I was 9, a teacher made me believe something that wasn't true for a whole day! Any more than they will do that because you told them about santa, or the tooth fairy, or the easter bunny or that ripping off a plaster isn't going to hurt, or that eating up their veg will help them turn into a super hero. Kids are not that fragile or, well, stupid.
The school are making a change by revealing at the end of the exercise that it was a hypothetical situation. It's a learning experience and the children will not go away from it distressed because the teacher told a fibby, but pleased because they got lots of praise for their fab letter writing.
At this point, over a year later, by demanding they follow a proper complaints procedure, by still banging on about it even though they reviewed your complaint and made changes, but they didn't do it In The Proper Way, you are just going to become, if you haven't already, an object of ridicule.
They don't want to stop the exercise. They clearly feel it has a purpose. They have made a change to it, recognising the point about letting children believe it for too long. That's enough. Just let it go. At this point, it is probably more about winning than anything else. If you were able to be honest with yourself.
Suppose you had them go through the whole thing according to the complaints procedure. Do you know what would happen at the end of it?
They would find that the school could carry on doing it.