The thing I saw about it is while it could work relying a awful lot on a single piece of rope and the bird coming in from one specific angle. It also depends on how the rope is held/tied. If held by the cadets they can be thrown off the building. If tied to the building, well, these are ruins. They might not be stable enough to hold. 

It'd work much better with a safety net in order to catch the Roc with more ease in case one didn't do as depending on the angle they could go under or even completely over the rope. Honestly if we had no other choice it'd be a decent plan but it can be made even better with amy one armed with a ranged weapon behind the bait for when the Roc sweeps. That way one can hit it easier and the bait has a bigger chance of survival and trap has a larger chance of success.