Here is one site that I found with plenty of neat Text-to-Speech tools. http://udltechtoolkit.wikispaces.com/Free+text+to+speech
I did download SpeakIt! for my Chrome browser (if you use Chrome as your browser you can add APPs, under extensions). In the settings for SpeakIt! I changed my text-to-speech settings (from my Mac) to iSpeech and a little icon shows up next to my wrench. When I want something spoken to me, I highlight the text and go to my browser and click on the audio button. I love it!
Well, I don't teach in a classroom but do have a free tool I can recommend:
Copy/paste website text into Vozme and the site will create an mp3 file that can be saved and played. Disadvantage...voice used is robotic.
Here is another - Blind Speak - http://beta.blindspeak.com/
BlindSpeak is a new text to voice email application that lets you convert text into speech and then forward it to any email address. The recipient will get both an MP3 file and a link to the online Flash player to play the message.