In a webmaster hangout, Google’s John Mueller said that you do not need to disavow sitewide links or blogroll links if they are organic and natural:
“If these are normal links, organic links, that are pointing at your content, then I would just let them be. That’s the way the internet works. People link to your content.
If your students have blogs and they think, oh, this is actually a teacher that knows what he’s talking about, then that’s a good link. That’s not something you need to disavow just because maybe it’s a sitewide link or in the blogroll.”
You can view the video here: