Our PEA cross country and skate ski class has begun, and we had great weather on the nordic track on Mount Hood last weekend. To video the students' ski technique at this first class, we didn't even need to use the waterproof case that was purchased for the iPad! The beginning cross country skiers got a lot out of being able to see themselves ski, and being able to compare that to an "expert" skier (i.e., their instructor).
There's a free app available thru the App Store called "Ubersense", a coaching application. It allows you to video a student doing an activity, go thru it in slow motion, freeze frame, and even draw on the video. You can record "coach comments" over the video, and send it to the student to look at via email or Facebook. You can pull up two different students on two different videos, and compare them on a split screen. There are demos to look at when you download the app.
While this app would be most applicable to the various PEA activity classes, it would also work for any physical activity you're teaching to students- I could see it being useful to record debate tournaments, or fieldwork techniques for geology, biology, or environmental studies. Or, to video student lab technicians you're coaching to do some laboratory work that involves some kind of physical dexterity. For example, if you were teaching some students how to use a pipette in your lab, you could video their technique with Ubersense, and then play it back for them, drawing in any correction in their technique that you'd like them to work on.
And, if you've got a couple little kids playing soccer after middle school, their coach would probably love seeing how this free app works. :)