Congratulations! You have written a post that for whatever reason has made its way onto the radar of comment spammers. You may not even realize that the comments are spam. I mean they are just complementing you and your post, “Great Post!! Thank you very much!” and “This web site is really a walk-through for all of the info you wanted about this and didn’t know who to ask. Glimpse here, and you’ll definitely discover it.” are two recent examples from sites hosted on Voices. These comments look innocent enough but it is the links used for the commenters URL that are the real issue. The commenter’s URL usually points to an adult site or a marketing site. So what can you do to stop such comments?
All sites on Voices already have some hidden comment spam plugins working to stop such comments, but these plugins are just checking the text of the comments for words associated with spam or excessive links in the comments. Notice that both the examples above seem like they could be legitimate and so the plugins checking for spam let them through. Dealing with comments like these involves either changing the Discussion settings under the Settings panel or enabling another plugin on your site.
If your site really only needs to get comments from classmates then you should be able to check the
Edit: Also, see WordPress.org for some more helpful information on dealing with comment spam.