This is such a critical piece of the successful running of a course. You can assume students know what to expect, especially if they've been at the school for awhile, or you teach more than one class and you've had those students before. But I find it is always better to take the time at the beginning of every class to set clear expectations, go over the syllabus and school policies, and establish clear boundaries. And, it's always better to be a little harder/stricter at the beginning because you can ease up later as opposed to being soft at the beginning and then trying to enforce structure later on.