Surely the school didn't purchase a bunch of new heart monitors because it might improve the calorie-burning of their students.
If you haven't been paying attention this summer -- fat people are the new terrorists. It seems a lot more plausible to me that a school is implementing a weight control plan than that they're expecting a gym teacher to diagnose cardiac abnormalities with a heart rate monitor, something a cardiologist couldn't do usefully.
Thinking this over some more, though, I'm more sympathetic to the asker's paranoia than I was at first. If school's can embrace policies of publicly weighing and humiliating children, they might well decide that the heart data might be shared in some inappropriate way, although the insurance thing seems unlikely.