It is likely this teacher's class is not yet connected to the organization. When pre-existing teachers join the organization, they will need to merge their class under the organization, as well.
To merge classes into the org, the teacher should log in and visit the following page:
Once connected to your organization (teacher account and pre-existing classes), post/comment capabilities will be restored. If you continue to experience issues, please let us know by emailing us at firstname.lastname@example.org.