In the campaign to eradicate childhood obesity, schools around the country have banned everything from birthday treats to vending machines. But when it comes to slimming down students, the so-called food police aren’t making much progress.

Now the Massachusetts Public Health Council has decided to take another approach: Body Mass Index surveillance. The new program will monitor kids’ BMI, and parents of heavy children will be notified with a special “report card.”

Sadly, this scheme won’t encourage healthy habits any more than turning cupcakes into contraband. But there is a better way. If Massachusetts officials are committed to helping students lose weight, they need to get kids moving.

The first problem with measuring students’ BMI is that it’s a poor measure of who’s actually overweight or obese. The BMI is a simple ratio of height to weight, used by governments to classify people as “normal,” “overweight,” or “obese.” The result is a system so flawed that it considers Tom Brady officially fat. (It doesn’t take muscle mass and fat mass into account.)

Even the Centers for Disease Control and Prevention openly doubts the effectiveness of BMI as a health promotion tool.

Though this so-called surveillance system is new to Massachusetts, the Bay State isn’t the first to experiment with it. With the intention of tackling its own obesity problem, Arkansas introduced a similar “report card” program in 2003. It failed miserably.

As The Baltimore Sun reported three years later, “The BMI testing has not put a dent in the state’s number of overweight kids.” Thirteen percent of parents reported that the program was a source of humiliation for their kids at school.

Instead of finding innovative ways to lower children’s self esteem, schools should look at what researchers have discovered about weight gain among teens and ‘tweens: It has little to do with BMI measurements, or even junk food.

Research published in February in the journal “Pediatrics” found that nearly a third of schoolchildren get little or no daily recess. The study’s authors suggest that childhood obesity should be attacked with more physical activity in school, where kids spend a majority of their waking hours.

The Pediatrics study also found that children who are allotted recess time in school behave better in class. And a month earlier, Harvard researchers demonstrated a clear relationship between physical fitness and academic achievement.

Compare that with the measly 10-minute recess at Peabody elementary schools, for instance. (That’s about how long it takes to pick teams for kickball.)

Lack of exercise is a problem that continues as students grow older. According to a 2007 CDC survey, 82 percent of Massachusetts high school students didn’t have daily gym class. Yet 28 percent watched three or more hours of TV on school days.

In a speech at the National Food Policy Conference in 2003, then-FDA Commissioner Dr. Mark McClellan said that “in a debate that has often focused on foods alone, actual levels of caloric intake among the young haven’t appreciably changed over the last 20 years.”

Rather than demonize birthday goodies or scold the parents of overweight kids, shouldn’t teachers be looking for a positive approach to help students manage their weight?

The difficulty is that feel-good legislation like Massachusetts BMI reports cards is easy but ineffective. Getting kids to move is a much more difficult task for legislators to accomplish, though it is possible.

Teachers at Mars Hill Elementary School in North Carolina are already leading the way by taking an “exercise first” approach to fighting childhood obesity. Their third-graders have nearly enough miles on their odometers to have walked across the state and back.

BMI report cards don’t work. It doesn’t take a report card to tell parents that their child is overweight; it takes two eyes. Massachusetts officials should employ methods that are known to help fight childhood obesity. It’s time to teach children healthy habits and give them enough time to do what they do best: play.