Alison, I sympathise with your situation. After 30+ years in the classroom myself I know well the pain that follows the 'upgrade' to the IT system that then won't run your tried and trusted computer program!
Try this on your iPad - it works on mine.
Try going to Settings - General - Accessibility - Zoom and switch it on (it seems to me it's 'off' by default).
Zooming is now controlled by 'three-finger' actions! (although the original two-finger pinch still operates as well, on actual photos though not videos, I find)
Double-tap with three fingers together and slide up the screen to zoom in.
Double-tap and slide down to zoom out.
Simple double-tap three fingers to return to normal size.
This works with videos. In fact it works even 'now', as I'm typing this here in Safari! It works on the 'home screen' itself! - it works in 'Notes'. It seems to work everywhere in iOS 7.1
So, get your video on screen - use three-finger-slide zoom to achieve required zoom level - play.
Hope this helps with your lesson plans!