People with severe disabilities- especially children- are often not set up with the proper behavioral framework or communication devices with which to be successful. People act out when they can't communicate and their needs aren't supported. Maybe I've just been spectacularly unlucky, but in the low income settings I've been in (3/3) the aides have carte blanche to run all over (and run off) teachers who try to put these things in place and run the show in general like it's just a paid social club for them. The nightmare you witnessed sounds very familiar- willfully untrained adults who only bother to act when it's an opportunity to react rather than de-escalate or support students properly in the first place.