I'm dubious of how many US high schoolers don't have data - many will be on a family plan. Not to say none of them are data-less, but I don't think this will become the next Snapchat based on one feature.
For instance, her first week with a phone, my kid managed to consume almost half of the shared plan for the month. So that phone now has a hard cap set on data usage, and was wi-fi only until it reset for the new billing cycle. For bonus points, much of that time was spent at a Luddite relative's house who did not have a home Internet connection (though leeching off the neighbors was barely possible).
That wasn't even a grounding. Parents may wish to lock down a kid's phone to the point where it doesn't do anything but communicate with whitelisted contacts or place a voice call to emergency services. Taking the phone away, the only sure-fire way to eliminate undesired uses, also eliminates the phone's use as a tattletale/tracker for the parents. Parents who are not tech savvy may not even realize that mesh networking even exists, and thus Jott may present a partial countermeasure to grounding restrictions.
I have also seen social posts wherein parents encourage the completion of chores by changing the wi-fi password and giving out one character of it to their kids per chore done. Teen-to-teen meshes sabotage that strategy.
It seems to be the European countries that have users willing to pick up "data-free" apps.