Each student will need a username but does not need to have a blog. You then invite the students, up to 10 at a time, to become readers of the blog. They will be able to comment on posts you write.

http://en.support.wordpress.com/settings/privacy-settings/

3 I would like my blog to be visible only to users I choose – Select this option to create a private blog. If you want others to be able to view your private blog (and add comments, if you’ve enabled them) you’ll need to invite them to be a viewer.

also this:
http://en.support.wordpress.com/adding-users/#invite-people-to-view-your-private-blog

I wanted to introduce my class to wordpress.com by creating a class page to increase communication for assignments. However, during a session with one student to create a how-to-guide, I became aware that each student would be forced to create a blog. This will overwhelm some of the students due to age, and others simply don't have time. Is it possible to have people become members and leave comments with out setting up a blog? In addition, I requested to follow the blog with a different e-mail. This blog never showed up and I was pushed to create a blog.

If every person must be a wordpress blog user, this will grossly limit my audience.