Open a lesson activity as a student and you will be able to see a 'Preview' link under 'Lesson administration' in the Administration block.

The link doesn't go anywhere.

In /mod/lesson/lib.php, there's some missing code.

This is what it currently is:

$url = new moodle_url('/mod/lesson/view.php', array('id'=>$PAGE->cm->id));
$lessonnode->add(get_string('preview', 'lesson'), $url);

This is what it should be:

if ($canedit)

I've tested it and now students cannot see the 'preview' link in the admin block but teachers still can.