It will be easier to recreate the situation by giving you access to our site.
URL: http://choose2change.com/lessons/mod/forum/view.php?id=31
Put in kwayne for login and kwayne for the password
this should get you to the forum page labeled
"AC-015 Addiction

• C2C
• / ? Addiction
• / ? Forums
• / ? A-1

Although the student is in the group he cannot reply to the original posting nor to any of the teacher replies. A "postnodiscussion" error message appears.

to view the site as an administrator
logout and then relogin as guestadmin with the password: admin
click AC-015 and then click A-1

at the top of the screen on the right you can see the settings for the forum by clicking "update this forum"

you can also view the group settings by clicking on "addiction" on the menu bar at top left and clicking on "groups" on left menu bar.

Hope this helps you to recreate the problem and more importantly provides enough information to solve the issue

Thanks for you help