That's an excellent mission! For younger kids, I really dig how PiFace integrates with Scratch. And Python is a nice next step when a student is already familiar with programming logic from Scratch. LEGO does already have the WeDo physical interface for Scratch: http://info.scratch.mit.edu/WeDo But I believe it's pretty expensive ($130) and requires a computer (or I guess a Pi if the USB interface is supported). I'm thinking PiFace+Pi would provide a cheaper route to do physical computing with graphical programming.
From the PiFace readme they link to Google Docs for a lot of nifty projects. For example, a ButtonBox: https://docs.google.com/folder/d/0B-UAZ9CyJCLGQjJ3RDlqa2pqaDg/edit?docId=0B-UAZ9CyJCLGaHI2T3NXOWlfLUE
I'd suggest shooting the maintainer an email, https://github.com/thomasmacpherson/piface, to see what more they have planned from an educational perspective. You can also use their contact form:
And finally, the official Pi Education Manual looks great!