Just my 2 cents
We have different issues involved around dances at our school because we're a very small PS-8. We've had complaints in the past about our dances being a free-for-all with the main problem being the parents doing the "drop and roll" (aka drop your kid off at the door and make a break for it). The little kids ran around all over the place, the big kids were out of control and it was just a bad accident waiting to happen.
When the new board took over we tabled the motion of a dance until further discussion. Much to the horror of the team, we discovered the VP lined up entertainment without consulting us, then we were uncomfortably in the situation of having a dance noone wanted to have in the first place. So, we made some ground rules. The VP ignored all of them (including that she was in charge of it) pre-dance until the week before. At that time we had a messy meeting, the basics of which were "you are in charge of this event. If you don't have adequate parental involvement, we're pulling the plug!" Within 2 days 15 parents were lined up.
With just a couple of parents hemming and hawing, we made changes and made it clear to the kids (and adults) that if rules were not followed there would NOT be a dance next year. Rule 1, each child must be supervised by an adult at all times. Rule 2, sign up at the door ensures rule #1. Rule 3, The dance is an event that takes place in the gym...not the bathroom, not the halls, not the parking lot. Rule 4, if you must leave the building for any reason, your child MUST be under the care of another adult. Rule 5, if you break the rules you WILL be going home.
We did have a couple of problems, but adequate staffing helped to nip them in the bud. (including sending a boy home immediately when he started wrestling in the hall) A big dad at the door and another posted down the hall virtually eliminated bathroom and exterior door issues.
For the first time in years we had parents actually saying they enjoyed themselves at the dance and they thought, bar a few minor glitches, it went off terrific.
But how do you handle a disorderly parent?
Someone (not just the President) should have called the police. The president of a PTO has more things than you can possibly imagine to do which makes her or him the absolute worst person to put in charge of crowd control at an event. It does appear that the PTO didn't necessarily address concerns ahead of time. A committee is needed to organize an event, but that doesn't mean that the PTO is totally off the hook either.
Finally, I am a bit confused about how things were allowed to escalate. Were there NO responsible adults at the dance? In this age of cell phones NO ONE felt the need to call for help sooner? While I really feel for the kids missing out, I gotta side with the principle on this one. The "good" parents did nothing to stop this drunken mom. We need to teach our kids by modeling responsible behavior, not just by NOT misbehaving.