Hello,
Do the exams have a time limit? If not, then students will be able to exit and return to the exam as often as needed, up until the "Available until" time that you have set in the settings.
If there is a time limit, then the test timer will start when the student begins the test, and continue to run in the background even if the student leaves the test. There is no way around this.
The closest thing you can do is to allow the student extra time on the test, so that they will have more time leave and take breaks and return if needed. There are instructions for doing this at
https://community.canvaslms.com/t5/Instructor-Guide/Once-I-publish-a-timed-quiz-how-can-I-give-my-st...
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