1) Create a site with either two sections or at least one group.
2) In Forums tool, make one Forum accessible to the whole class and another restricted to the group or section. Make sure to set the Student permission to None and the group or section to something like Contributor.
3) Log in as a student who can post to restricted Forum. Post to the restricted forum and the whole class Forum.
4) Log in as a different student who does not have access to the restricted forum. Go to the post you can read. Indicate you have read the post either by clicking Read All, or clicking the envelope icon with the post.
5) As instructor go to Statistics and Grading.
The result is that if you look at the student who does not have access to the restricted forum, it will show 1 read and 1 unread, for 50% read. It should not count against a student if he cannot read posts in forums or topics from which he is restricted.