Cornick was last week sentenced to life in prison with a minimum of 20 years after he killed teacher Ann Maguire at Corpus Christi Catholic College in Leeds.
Richard Wright QC, who represented the teenager, told The Observer that the decision by Mr Justice Coulson to name him has placed him and his siblings at risk.
He said that decision had gone against expert advice that said he would be targeted by other inmates in jail.
Cornick’s family would also suffer, Mr Wright said, and his parents’ ability to support him would be affected.
“Will Cornick has two siblings. Where is the protection for them? They are just as much victims and just as likely to suffer from being named,” he said.
The court’s decision to no longer apply section 39 of the Children and Young Person’s Act 1933 had allowed Cornick’s identity to be revealed, but Mr Wright said even if it had been enforced his name would still have got out.
“Section 39 has limited effect because of the way it’s drafted,” he said.
“It was written before websites and bloggers, and therefore has no bite on the internet. The mere fact of naming him on social media was not in contravention of the court’s [anonymity] order, and that’s obviously a problem.”