In order to prevent a student from accessing an assessment when they are not intended to, if you are using Mastery Connect student experience (student.masteryconnect.com), then expire the TEST ID or set a custom TEST ID expiration time. If you are using the Canvas LMS + Mastery Connect integration, you will need to ensure that the availability window is restricted to when they can access the assessment. Keep in mind that if a student does not close the window in the Canvas LMS experience, it will not restrict them from submitting/accessing the assessment. This is a known limitation of LTI Integrations.