One of the main reasons I will be refusing is that they do it at 11 or whenever it is they leave primary school as well as when they start. I dont think its going to make a massive difference at 5 ish years old but at 11, it really could depending how its done (whether they are big or small) and I just dont trust them to do it sensitively.
There was a girl in my class at 12 years old. We got weighed and measured, she was on the slighter side of average and she sort of got "praised" for being perfect by the school nurse. It triggered something in her - the validation - over that summer she became anorexic and when term started the following year she didn't come back for a while.
She wrote us a letter saying she had been in hospital with anorexia but was hoping to return soon.
She did eventually return looking okay but she said that it started when she got weighed and she liked being told she was perfect and wanted to be "more and more perfect" and loved getting on the scales and seeing the numbers going down further towards the "ultimate perfect". She'd never really noticed scales before that day.
So its not just a case of "protect the fat kids from potential humiliation" which makes me think that being weighed in a group in what could seem like a competitive situation could be at worst, damaging and at best, unnecessary when most homes have a set of scales and failing that, parents and GP's have a set of eyes which they can follow up with some scales if necessary.
The ones who "slip through the net" may well do so anyway - parents who are not willing to accept their child is obese probably wont anyway - they will say BMI is rubbish, the child has big bones, they are built that way, and wont be "told" by a letter anyway.
I just dont trust them to get it right, and feel its my job, not theirs, and if only to protest that point then its worth refusing consent for me. They can only overstep the mark if you let them.