YouTube has unveiled a site where they are only placing educational videos and where comments will be disabled. The site — YouTube.com/education is supposed to be able to be allowed through content filters even though YouTube itself is blocked.
You can read more about it at the MindShift blog.
In the meantime, you might also want to check out The Best Ways To Access Educational YouTube Videos At School.
Sadly, the education site is still blocked at our district. Sigh. There are ways to get around the block, and hopefully with a new system coming into our district soon, teachers will have access to youtube, although students will not.