The school districts that are doing it correctly are the ones that are collecting the data anonymously, no names connected to the data. This kind of aggregate data can be helpful and is not personally intrusive.

Why would anyone be against this when it works to reduce childhood obesity where it is being done?

"To take from one, because it is thought his own industry and that of his fathers has acquired too much, in order to spare to others, who, or whose fathers, have not exercised equal industry and skill, is to violate arbitrarily the first principle of association, the guarantee to everyone the free exercise of his industry and the fruits acquired by it." --Thomas Jefferson