Consider - an app that replicates this functionality on the phone, but tracks if the user at any point closes the app. This then is reported to the teacher so the teacher knows if there was any cheating.
This simple change will improve the quality of education and reduce the cost of calculators.
They're mad useful for learning - you can explore maths very effectively, but there's a learning curve to it.
I started using GraphNCalc83 on my iPhone (maybe available on Android - I'm sure an alternative is if not), and I question the real need for a £120 calculator.
The calculators are a completely counter-productive and distracting tool for “learning” which have contributed to a watered down curriculum with more mindless button punching and less thinking than before. Anecdotally some students never recover from the mistaken idea that math class is about learning how calculators work.
There is no reason to assign problems which require a calculator in high school mathematics courses, and for anything that would benefit, there’s a huge advantage in using a computer with a full-sized keyboard and a general-purpose programming language, or even a smartphone with a web browser. Do your basic plotting at the free desmos.com, with the added bonus that sharing plots with classmates is trivial because you can generate a URL.