There is nothing you can do from preventing your students from taking screenshots. That being said, here are some suggestions:
1) Create a large Question Bank so that each students gets a different, randomized Quiz.
2) Allow your students to see only one question at a time, and lock each question so they can't go back.
3) Set the timer and give the Quiz at the same time for all students. (Tuesday 10:30 to 10:45 a.m.)
4) Divide larger assessments into shorter Quizzes and put them on a schedule: (Part A 10:30 to 10:45; Part B: 10:45 to 11:00, Part C: 11:10 to 11:15...)
5) Show the students the correct answers at a later time or day.
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