You can set forums in the Discussion and Private Messages to be accessible to a specific group. When this is done, only students assigned to that group will be able to read and post to that forum.
To assign groups to a forum in the Discussion and Private Messages tool, click the Manage link shown at the top of the Discussion and Private Messages area. In this example, a forum for Group 1 was already created. For more information on creating/editing forums, click HERE.
On the Manage Screen, click the Forums link from the menu on the left.
Since we are assigning Group 1 to the "Group 1 Discussions" forum, click on the "Click to Edit" link to the right of "Group 1 Discussions."
In the list of settings for the forum, select the radio button next to "Allow Access to Selected Groups." The list of groups available for your course will appear below. Check the box next to the group you would like to allow access for that forum.
Click the "Update" button at the bottom of the settings and your forum will then be accessible to only the group you selected.