Would Arduino be an appropriate platform to teach high school students?
Given the relative inexpensive cost of the arduino and all of its available variants, I think it's a perfect platform.  What we're finding is that when learners are engaging more of their senses during the act of learning, they're more likely to integrate the concepts.  Since the arduino really connects the physical world to the digital world, it's awesome for cementing the abstract skills of programming an LED to turn on with code, and actually seeing it happen in real life. 

Please do. You can get them at Radio Shack now, and they're well within reach cost-wise.  Also: open source and cross-platform.  The Arduino is a winner all the way around for beginners.