I'm betting it's not even that and it's just a heart rate monitor to improve the quality of aerobic exercise. I concur.
I worked as a technical writer for a place that made HRMs. We sold to pro athletes, gyms, personal trainers, Navy seals, fitness enthusiasts of every stripe... we even had a version of the product made especially for training race horses. It was pretty cool.
I was surprised at what a difference using one of those things made in my *own* ability to exercise. I'm an overweight writing nerd, but man: there's nothing like beeping, booping technology to get my interest. Using an HRM is like keeping score on a video game. Or playing the tomagotchi game with your body as the avatar. Or something.
Something fun and trackable, anyway.
The HRM went a long way toward getting me off my butt and dropping pounds because it provided metrics and feedback that I could understand and affect. That's more than my "hustle! hustle! hustle!" school coach ever managed to do.
All this being said: I doubt that the information on your kid is going to be recorded for more than 9 weeks, honestly. There are, like, serious LAWS about that information getting off campus, too. Anybody who is into selling kids' info to Nefarious Businesses Incorporated is going to have access to a lot more dirt than just a weird blip on your child's HRM.
That HRM, by the way, is certainly *not* medically diagnostic in quality. I'd be surprised if it did more than note the heart rate at 1 second intervals and track the changes over time. It *might* try to estimate a general sense of fitness on the heart, but it will, at best, give you a meaningless number on a scale from "is this thing on?" to "cybernetically enhanced athlete trained atop the Himalayas from birth."
No need to worry. The poster's school's coach is probably just trying to do a great job at keeping the kids in his care interested in physical fitness. I applaud him/her for it.