The teensy is...well...tiny!
It doesn't have the same footprint nor is it pin compatible with the Arduino (meaning no shields). Also, being a different controller, I have no idea how (or if) you program it with the official Arduino IDE. It certainly can't be as functional as the official Arduino.
Do your friends know how to solder? It could be a fun part of the class:
http://nkcelectronics.com/freeduino-arduino-diecimila-compatible-board-complete-kit.html
That comes out to ~$19 each (at the discounted rate for 10), and it is 100% compatible with the original Arduino.