Yes, it worked for you - but surely in an adversarial context, there's absolutely no way to trust a previously untrusted device. In the extreme case, the entire interface might simply be a simulacrum app designed to feign success at locking.
Yup, a student could easily write an app that fakes the Desmos Test Mode confirmation screen, just like how a student could write a TI-BASIC program that displays "MEMORY CLEARED" on the screen. What's old is new again...
Anyone can install up to three custom apps on an iPhone from xCode using a free Apple ID. The catch is that they will stop working and need to be reinstalled after seven days, so not useful for every day use, but perfectly fine for testing or, in this case, mischief.
I don't know how that would work since the teacher could reboot the device or hit/swipe home to exit the fake app. If the fake app was in guided access mode, they'd see a pin input instead of a home screen.
The only way to fool the teacher would be to jailbreak the device. The 6s running iOS 9 is the most recent untethered jailbreak. All newer devices and iOS versions will revert back to stock kernel after reboot. That means no jailbreak features and the home button & guided access mode work normally.
So to fool the teacher you'd need:
- An iPhone 6s or earlier
- Running iOS 9 or earlier
- With an untethered jailbreak (Pangu9)
- Running a fake app that behaves just like the app
- And behaves like iOS's guided access mode
I think that's difficult enough to discourage the vast majority of cheaters. If the school really wanted to, they could hand out loaners to the students with old iOS devices. They'll need a bunch for the students with and Android devices anyway.
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.)
Why does the Jailbreak need to be untethered? Is the teacher walking around and making every student reboot in front of them? Rebooting takes a solid minute, so in a not-particularly-large class of 20 kids, you’re dedicating a solid 20 minutes to this activity.
Once Jailbroken, I don’t think you’d need a fake app. Just make a very simple patch that overrides whatever is preventing you from returning to the homescreen, and set up the patch to be enabled by a secret Activator action, like pressing the volume buttons in a secret sequence.
There's actually a lot easier ways of cheating on tests than that even. Teachers still have to watch their students, the solution is just designed to reduce the temptation for the average student to cheat.
