peakd is a free Google Docs add-on that will read your documents aloud. When you have Speakd installed in Google Docs you can open the add-on and press play at any time to hear your document read aloud. Unlike some other text-to-speech tools, Speakd doesn't require you to copy and paste text to hear it read aloud.
Applications for Education
Voice that Speakd uses is very robotic. That said, Speakd could be a good tool for students to use to hear their documents read aloud as part of the editing process before turning in a final submission.