1 - Go to Google Video - http://video.google.com
2- In the search box, type: genre:educational
3 - Follow it with anything you want to find video for.
eg. I typed genre:educational computer science
You can type genre:educational chemistry ( just follow my hyperlink) and there are some AMAZING chemistry videos from college professors. Talk about showing students what college will be like, how about letting them hear a lecture from a college prof, maybe they'll appreciate you for a change!