Do schools' BMI screenings of students even work?

I remember opening the envelope holding my daughters' report cards from their New York City elementary school a few years ago and stumbling upon a fitness report. The so-called "FITNESSGRAM" tallied my daughters' height, weight and body mass index, or BMI. I had no idea we would get such a report or that my daughters were weighed at some point during the school year to come up with the measurements.