I don't mean this rudely at all, but I am slightly confused about what aspect you are objecting to (and I think some respondents might be too).
Is it asking then to weigh themselves in the first place? If so, I take your point - but this almost certainly isn't the teacher's decision, she will be following the school's approved curriculum. So yes, complaining about this might well be appropriate, as it may not be an issue the school have considered before.
I don't however think the teacher did anything wrong by making a suggestion to the kids to put down an average weight if they didn't want to write the real one. That sounds like she was attempting to avoid any potential issues of bullying/shaming individuals.
As a teacher myself I know how stressful and upsetting parental complaints can be. That doesn't mean I think parents should never complain (and I have even done so myself on one occasion). Bur I would want to be very sure the teacher had acted with deliberate malice - and I don't think this one has.
The most important thing you can do is to educate your daughter about self esteem, societal pressure, healthy bodies etc etc. Which it sounds like you do. I highly doubt this one lesson will do any damage. Nontheless, flag it up as a curriculum issue if you feel strongly.