Closely inspect the cable. Don't move it. Just look at it. Is the cable at all bent, does it have individual strands that are pulled out or damaged? Is there any part of the cable that looks like it's been bent, kinked, damaged, or in anyway doesn't look normal. Cable get's it strength from having a lot of tiny fairly weak cables wrapped in a perfect
pattern. If anything happens to damage the way those cables are wrapped together it results in a massive
drop in the strength of the cable. If there is any
clear sign of damage to the cable I would have the
stage shut down until repairs can happen. Even if there isn't any obvious physical sign of damage the cable may still be damaged internally, or it may have been stretched which would be very hard to see.
There definitely is a danger no matter how much damage you may see. How dangerous, it's hard to tell without a professional looking at it. Remember the
track and/or pipe that the curtain hangs from could easily kill someone if the cable failed. How much does your school value the lives of the people on
stage? Are they willing to gamble the lives of their students and faculty by not repairing it? No matter what the appearance of the cable, it needs to be repaired soon.
The good news is you live near a major city so you should have plenty of repair options, the cable itself is fairly cheap to
purchase, your most expensive factor will be the labor.