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.
I love how the first answer that HN proposes to a social problem is 'more capitalism'.
Why not... Just not use graphing calculators in high schools? What's wrong with pencil and graph paper? Not a single one of my algebra or calculus courses ever used the TI-83 for anything that I couldn't do by hand, or with an $8 calculator.
There is zero reason for why high school test questions should ever require a smartphone, app, calculator, or any other electronic device.