Yes, that happened about 30 minutes from where I live at Waterworld Concord. As you said, the kids did a chain, one stopped partway down the slide, and the rest in line bombarded past the lifeguard to make their 'longest chain' in the history of their high school, which resulted in collapsing. It happened the same day my school did their 6th grade trip to what was then PGA.
But yea...where are the lifeguards? Epic fail on their part.
If you were the lifeguard/ride-attendant at the top of the slide and 60 kids run past you, there's nothing you can do. Supposedly you shouldn't leave the area you are at, and the only way you can stop them is by physically blocking them from getting on. But seriously would you put yourself in front of 60+ kids running at you to get on the slide? I wouldn't. You would get run over.