Posts already shared can be edited. To edit a post click on 3 dots grid on right side of any post.
There are 4 options available: Move on top, edit, delete and copy link.
Move on top: for moving the post on top of the stream tab irrespective of the date and time of the post
Edit: editing the content and attachments of the post
Delete: to delete the post from the stream tab
Copy link: If teacher wants to share the current post on some other place in the same class or another class.