Can you do the same thing site-wide so that this will be the case for all forums?
1. Log in as site admin and navigate to Users --> Permissions --> Define roles.
2. Click on the link to the student role.
3. Click on the Edit button.
4. Scroll down to Rate posts and tick the allow radio button (the second column).
5. Scroll down to the bottom of the form and save changes.
Thanks,
Art