Thanks for the replies, chaps. These are 100Mbps Laserbit units. I've tried disconnecting the far end, and even tested the POE injector from the other building in the main building with the same results, I admit the symptoms seem to suggest a loopback issue, but i've isolated it to just that link so it must be either a faulty cable, or the laserbit unit itself. I'm going to try and bypass the cable next week during half term to see if this works, if not, it must be the laserbit unit itself causing the problem, would you agree?
As for pupils sabotaging the system, it does raise an interesting point doesn't it, were a special school so while I have doubts about our kids doing this, you should never say never, as the saying goes!