A report from independent reviewer Nick Page found that: "No individual other than Will Cornick should in any way feel responsible for Ann's murder.
"What is clear to me is that no one could have predicted or pre-empted Will Cornick's attack on Ann Maguire and, following her murder, individuals and organisations acted courageously, coherently and professionally in supporting the school and affected people."
Although Cornick had threatened to kill Mrs Maguire in social media postings, Mr Page said there were no warning signs to staff or other agencies at the time.
The report told of how the teenager had had a happy childhood with parents who were loving and supportive to their son.
The school had praised the boy and Mrs Maguire wrote about him in Year 10: "William is a bright conscientious young man."
The report revealed something changed in December 2013 when Cornick "communicated on social media to a friend about his hatred of Ann and talked about brutally killing her".
Then, in February 2014, there was a dispute between Cornick and Mrs Maguire over a detention and, later, his parents said his relationship with the teacher had "broken down".
Mrs Maguire was "baffled" by the news, the report said.
Cornick then sent another message to a friend on Facebook, saying Mrs Maguire "deserves more than death more than pain, torture and more than anything that we can understand".
Although the boy made more statements about he was planning to do to Mrs Maguire and another pregnant teacher, pupils did not take them seriously, saying they had heard them before and that "Will had a dark sense of humour".
The report said: "There were no credible 'warning signs' - behaviours or antecedent behaviours - that could or should have been picked up by agencies or professionals leading up to the murder."
Mr Page concluded: "This was a unique event and there are no recommendations I can make which in hindsight could have assisted staff in predicting or preventing the murder of Ann Maguire by Will Cornick or in preventing a similar event in the future."
Commenting in a statement on the review, Mrs Maguire's husband, Don, said it "appears to be significantly different from an early draft report which we viewed some months ago".
He went on: "Therefore our family shall need some time to read and consider its findings.
"Once we have completed this we will make a statement and I shall make myself available to answer questions or for interview."