An entire site can be set to private. This is ideal for an Intranet of a large school or department. The Site Admin can do this by going to Settings > Reading : Site Visibility.
Having a private site will limit the access to the site/pages and any media files (images, documents and etc.) within the site. A user is required to login to view the private site and will be prompted to do so when accessing a media file directly. When a site is set as private, all media files are moved to a private folder thus changing the URL.
Keep in mind that these sites do not meet legal standards for security.