You can decide how students can post and comment in Classroom. You can set permissions by class or by student. If you allow posting or commenting, students can't edit their posts or comments, but they can delete them. Teachers can delete any post or comment.
Post—Information or a question on the class stream. Example: When do we visit the museum?
Comment—A response to a post or comment. Example: We go next Friday.
Reply—A response to a comment that mentions the person who made the comment. Example: +commenter’s name Thanks!
Even if you don’t allow students to post or comment, they can still send you a private comment on an assignment or question.