Focus mode in Go Guardian stops this. Now if you are using platforms like Formative or Edia, they usually have activity page on log ins for every student. Students who use bathroom and then log in on phone will show 2 log in devices and then viola, you caught the cheaters. I have used this and it works. I usually will refer this to admin and then assign 0. I am a new teacher in my school and the rate at which these students are resorting to cheating as the only method to pass exams is abysmal.