Teachers can set any students that are in Groups that they own to read-only status so that the students can read what is going on in the Group, but cannot Post to the Group. Students on read-only can still send Direct Messages to their teachers. This can be turned on/off for individuals at any time. Learn all about read-only status here.
Teachers can also moderate Posts in their Group so that they must approve all Posts before they go live in a Group. Learn more about moderating Posts here.