Create a YouTube School account http://www.youtube.com/schools and guide your student access to appropriate YouTube content. Many videos from recognized educational sources (including Khan Academy.)
"School admins and teachers can log in and watch any video, but students cannot log in and can only watch YouTube EDU videos plus videos their school has added. All comments and related videos are disabled and search is limited to YouTube EDU videos."
Available also are playlists created by other educators. Videos are aligned to common educational standards and organized by grade level and subject.
YouTube also has created a page for teachers, YouTube.com/Teachers, to help them learn to use the site as a powerful educational tool.