lock up all the exacto blades and other shape objects.
Seriously would have been a different story if your equipment would have gotten involved in the fight or broken, and the idiots getting injured on something on your property......
Maybe start looking more closely at your employees. Do urine test for drugs if needed. Do background checks before hiring, watch their moods, and talk to your lawyer and insurance guy. You definitely don't need slapped with a negligent hiring or negligent supervision lawsuit.