We have letter grades on school report cards why not BMI rankings?

Why shouldn’t schools put a kid’s BMI rating on the report card? People want to know just how much their child is learning so why not how fat or healthy their child is? People don’t like to see it almost as if they are scared. They don’t just mark on a report car “passing” or “failing” you get a letter grade so the parents know well or poorly their child is doing. Health and weight wise they should be given the same information so they have the chance to do something about it before “Sparky” tips the scale at 380lb or better at his middle school dance.