While there is some educationally relevant content on YT, there is also plenty not appropriate for the school setting. Also sad that many videos have comments polluted with inappropriate comments (profanity,etc).
Would love to see sites like TeacherTube and SchoolTube become the defacto education video sharing site instead of YouTube.
It is blocked in my district.
you might try http://www.youtube.com/k12.
If kids have access to youtube at home, you could recommend videos for them to watch.