Somewhat like Google legitimately wants to "do no evil"?
Sometimes it is said that deeds, not words is what matters. But in this case even the words are missing, so I don't really get what you're trying to say.
Why can't the school just have a pile of them? Then they can be totally FOSS. The teacher hands you one, they know the FW is good, you give it back, and if you want to practice at home, you can buy yourself the unlocked version which would be fully open and have more communications so it could be repurposed.
Then the same device can be sold as a smarthome IoT remote like a better Logitech Harmony, or various other random uses, and have more economies of scale.
In fact, it could just be a cheap Android device sans GPS, with no front camera(A cheap rear camera is always good for QR setup) and with real buttons and a smaller screen.
Lower resolution + transflective display + 30hz + fewer background apps is going to significantly increase battery life.
Then people can try out the CAS features on their phone, and use their calculator for whatever they might want, since it's become the obvious choice for IoT stuff.
This is a viable issue that forces a choice on the manufacturers, which you aren't addressing. Of course it would be ideal if they had a second version (perhaps in a different color) that was fully hackable, and had some hardware difference, but that would be costly for them and hard to police.
Cheating is sadly a massive problem. I was shocked to learn from friends who did architecture in college that smart students would hide their final project models (physical models of buildings) inside ceilings and the like in the days leading up to presentations and judging, to prevent rivals from smashing them.
Then making one that wasn't certified for exams is exactly what they should have done. As it stands, they're just another TI.
I don't think "just another TI" should be undervalued—I remember how exciting their calculators were in my youth, and now they're, well, there's an XKCD for that (https://xkcd.com/768). Some competition that would get them back to make a real investment in innovation would be very welcome, even if it didn't result in an open-source calculator.
Personally, I'm hopefully never going to take another standardized exam in my life - I'd like to see a graphing calculator that doesn't attempt to get certified for exams or school use, since this seems to be such a significant hurdle. But I know I'm in the 0.1% of graphing calculator users who don't care about AP/IB/the SAT/whatever.