[@Card Captor] LOL their sole purpose is retrieving the pen. Once they get your pen, they leave you alone, unless of course you try to take it back from them, which then, they become a little more aggressive. And their prosthetic eyes probably use some of kind of signaling technology. They can tell when a pen is no longer working based off an electronic signal between their eye and the pen. So they definitely know. 

BTW, wat an asshole classmate for doing that, whomever he/she is. haha