Schools definitely need to take a proactive role in the health outcomes of its students. However this role should take place as a guiding role by providing children with positive health experiences that incorporates good nutrition principles and opportunities to participate in physical activity that is non competitive and conducive to life long habits.
Measuring a childs weight as the only measurement of a childs health status is potentially harmful in a psychological manner.
I believe schools can play a major role in the management of childhood obesity by incorporating health for all children on a daily basis. If this can't be achieved outside of a curriculum system then it must be incorporated as part of the curriculum.