What more could have been done to avoid it? Well the School following the Government guidelines for such trips with that age group of kids would have prevented it totally from happening.
Two weeks before this happened I went on a school trip to a theme park, as park of their risk assessment we were all given strict instructions on what was allowed, one adult to every six students, and students were not allowed on specific rides (such as Rapids and large rollercoasters) unless accompanied by an adult in that ratio, I spent most of the day riding the Rollercoaster while another parent spent the day on the Rapids, at the time we scoffed at the reasoning behind it, as nerds it made no sense to us, it made sense a few week later.
There is plenty of blame to go around, but if you'd read any of the numerous articles on the subject, you'd be forgiven for thinking the park was considered to be 100% at fault.