Create a page on your website and only allow limited access.  You might only allow district staff members who are logged in to see the page.  You might only staff from your specific school.  Or, you could create a unique group by only giving access to individual users that you add one-at-the-time.  This could be used to implement pages like shared lesson plans or resources for a certain grade level.  The page you are on now is an example of this.  If you log out, you will not be able to see this page because it is set to only allow district staff members to see the page.

This tutorial assumes you already have the assignment app.  It begins with the assignment app on your page and demonstrates how to create a new assignment and attach a file to that assignment.