Personally I use Final Cut Pro, but I would agree with J_Miles. Your video quality is very reliant on the footage you shoot. With good footage, I have seen some awesome videos made on iMovie.
Check out Paul Lazarus's video on making chairman award videos if you havent allready.
http://youtu.be/7OLsjhk-VN8
What ever video editor program you choose, make sure to have students make practice movies to feel comfertable with it. You dont want the first time your figuring out how to use a feature be when your trying to make your grade 'A' chairmans award video.