Like millions of other American parents, my husband and I received a warning letter from the BMI police last week. Our active, healthy 8th-grade daughter is "very lean," according to her school-administered "Fitnessgram." The national student body monitors think this is a public health problem. The obesity report card lectured that because our daughter's body-mass index is "very low," we "should make certain" that she "is eating a healthy diet that includes the appropriate number of calories." Thanks, geniuses. We had no idea such measurements were taking place, and we wondered whether we were alone in objecting to this unsolicited...