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 within the site. However, any images, documents, or other media on the site will not be private. It is possible for users to share the links of those items directly, with no login required. This feature is appropriate for sites that wish to limit access to certain content, but do not require things to be secure in a more rigorous sense.
Keep in mind that these sites do not meet legal standards for security.