A public school mistakenly believed that a father had already been told about his son's death before it posted an announcement online.
As the teenager was being taken to hospital, his father, Christopher Avery-Wright, was contacted but at that stage was only told that his son had been injured. But as he was being driven by police to East Surrey Hospital, he received messages of condolence via text and email on his mobile phone from people who had read about William's death on the school's website.
William's mother, Lisa, had already arrived at the hospital but she was advised not to break the news of his death to Mr Avery-Wright until he was there too.
The Roman Catholic school, which also sent out emails informing parents of William's death, said it published the information in "good faith" but that its initial belief that both his parents had been told about the tragedy turned out to be "incorrect".
The school said in a statement: "What was published on the day of the accident was done in good faith and in the belief that William's parents had been informed of his death. Sadly, it transpired the school's understanding was incorrect."
William's mother criticised the way the school handled the matter, saying such news should not have been broken in that way. Mrs Avery-Wright, 43, from Crowborough, East Sussex, said: "It's just quite shocking to learn news like that in such a manner. It's not the way that you want something like that broken to you.
"We weren't happy initially with the way the news was broken and not happy with the way the whole incident was handled, and very unhappy that there was no safety net for the children as they were unsupervised. To me it's something that could have been avoided."
The teenager's parents are now suing the school for alleged failings in its duty of care to William, claiming he should have been supervised by an adult as he was making his way to playing fields.
Worth School's statement added: "The school attempted last year to resolve a claim under the Fatal Accidents Act, but unfortunately that remains outstanding. The school is unaware of any further claims."