[those were the days]

I too am in two minds.
I seem to remember being weighed at school, and to tell you the truth i presumed they still did it. Nitty Nora should make a comeback as everyone has had the sceniero of clearing the blighters only to be instantly reinfected.
Back to the weighing though..
I think that rather than just sending letter stating the obvious - your child is overweight blah blah blah.. (because majority of people know if their child is overweight or not, unless of course just on the borderline)
They should send helpful information and contacts to help them help their child, with details of kids clubs run by various sporting things such as swimming or athletics, and maybe a support person to talk to for advice, not just accusation.

If done tactfully, confidentially and provide decent assistance, then yes it could be a good idea to tackle the rising problem of obesity at an early age. If just standard letter stating the obvious etc, then leave the kid alone, they`ve enough time to get hung up about the way they look, allow them to be innocent happy children.

Finally, the charts used for guidelines of weight should be reviewed into sensible charts, not the kind to be considered normal you`d have to be like twiggy! but take into account how the child looks, muscle build, skeletal frame etc not just numbers on a piece of paper and scales.