At least in the typical Desmos Test Mode use-case I've seen, the teacher never actually touches the device -- the student just holds up their phone with the con

At least in the typical Desmos Test Mode use-case I've seen, the teacher never actually touches the device -- the student just holds up their phone with the confirmation screen when they hand in their test. Yes, the teacher could do a more elaborate test sequence to verify that phones haven't been tampered with. But that requires (a) a bunch of time per student, and (b) an in-depth knowledge of technology and eye for UI details that Hacker News commentors have but schoolteachers often don't. (Also, I'm not sure a jailbreak would really be required. On an iPhone X you could probably disable the Home-swipe gesture by setting that orientation to landscape or upside-down, drawing the UI and a fake home button right-side-up, and setting your app to "immersive mode" or whatever so the real home button auto-hides.)