YouTube is pretty hit and miss (but mostly miss) for specific educational clips for your students. For an alternative, try BrainPop and BrainPop Jr. for short, specific, age-appropriate, fun and educational videos to kick off a new topic or reinforce lessons and learning.
This site is a big hit with students for an excellent balance of humour and learning and a sure hit for teachers with its excellent content. I love how you can search the videos by standand, subject and grade too! The IPad app is also used by my two primary kids too! The Game Up section is my next area to explore!