If your school has a zero cell phone policy, you can prevent students from signing in with their mobile phones by following this guide.
When a student tries to sign in on a mobile device, this is what will show on their screen:
This feature disables students from signing in using an iPhone, iPod Touch, and Android phones (both app and web browser). Logging in using iPads, laptops, or computers is still allowed. Digital IDs still work on disabled mobile devices.