It seems to me that this is a continuation of an ongoing trend in our public schools of combining healthcare and education. Currently, in grade school at least, they do free vision and hearing tests, and at some point they check for scoliosis. With the exception of scoliosis screening, these are all healthcare related services, which may have some impact on a child's learning ability. The inclusion of physical education in the curriculum means that an interest in the child's health is within the mission of the public school system as well.
I think that philosophically, a well rounded, holistic approach to education would be best. Perhaps the future of public schools would combine complete healthcare with education so that the mind and body are treated equally. This would necessitate a change in focus and allocation of resources. I don't think that teachers and the school nurse or gym teacher are qualified to assess and make recommendations on a student's body mass index. It's a half assed approach at best and likely to traumatize more students than help. After all, what are they supposed to do? Most schools have only one gym class a week and the cafeteria food is usually of marginal nutritional value and not conducive to losing weight.
Our current obssession with standardized testing means there is no time for this kind of approach. I think it would be a Good Thingtm to pursue and make for a happier, healthier, better educated, population.