When enrolling students to a course section, there is a small box for you to check to restrict the student to seeing only his/her own section and responses from other students only from his/her section.  If you miss checking this box upon enrollment, the only way to enable the feature is to go into each individual student's profile and make the change.  This should be an option on the "edit section" page.

      

Comments from Instructure
Sometimes, with the volume of ideas we have coming in, we miss a duplicate.  This idea has a duplicate with Course-Level Setting: Restrict Users to Their Own Section(s).  Both threads will receive the same updates.
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