Non edition teachers are not allowed to respond to topics on hidden forums.
They can add new discussion topics, and the can see all the messages, but they can answer previously sent messages.
I has been playing with permissions but no way to solve the problem...
Moodle Docs explain that hidden forums are the way to create teacher forums. Above I copy and paste...
Prior to Moodle 1.7, each course had a teacher forum, accessible via a link in the course administration block. During the upgrade to 1.7, empty teacher forums are deleted. Teacher forums containing forum posts are moved to section 0 of a course and are hidden.
A hidden standard forum may be used if a teacher forum is required in a new course.