There's useable stuff on youtube for the classroom, but it may take awhile to find it. I was impressed with the quality of this clip. The audio was very clear in comparison to alot of the other things I saw. While this is a dry, boring clip (?), if you want to learn something about the brightness command in Photoshop CS3 it's great and short!
I found alot of suggestive titles, some of which were blocked thank goodness. I think if a teacher is to use a youtube video you've got to find it first. No letting kids look for stuff as a class project. I'm sure they'll look enough at home the way it is. Remember it's free and that's what you get!