This isn't really a programming solution but you should check out this webapp:

http://www.hemingwayapp.com/

It may not be the exact thing you're looking for but it can probably be helpful to your students.

I would also look at Python NLTK. I've only dabbled in the toolkit, so I'm not sure if it has what you're looking for exactly, but it's worth a look.

http://www.nltk.org/