You can require that students reply to a discussion before the see any of the other students' replies.
Open Discussions
In Course Navigation, click the Discussions link.
Add Discussion
Click the Add Discussion button.
Enter Discussion Details
Enter a topic title [1] and discussion details [2].
Select Discussion Options
Select the Users must post before seeing replies checkbox.
Save and Publish
If you are ready to publish your discussion, click the Save & Publish button [1]. If you want to create a draft of your discussion and publish it later, click the Save button [2].
When your discussion is saved in a draft state, you can return to the page and publish it at any time by clicking the Publish button.
Student View Discussion
Students will see a 'Replies are only visible to those who have posted at least one reply' message when they view the Discussion topic.
In Discussions Redesign, students will see a 'You must post before seeing replies' message when they view the Discussion topic.
Note: Sometimes students will work around this requirement by deleting their posts. You can change your course settings to keep students from deleting their posts.