She died after being stabbed while teaching a class at Corpus Christi Catholic College, in Leeds, in April.
Earlier this week Cornick pleaded guilty to her murder and was sentenced to at least 20 years in prison.
The judge said the teenager showed a "total and chilling lack of remorse" and may never be released for what he described as a "truly grotesque" act.
According to the Daily Express a friend of Cornick has been expressing his guilt over the incident.
Quotes attributed to the unnamed boy say: "I think that if I had told somebody he had had a knife or that he had said he was going to kill Mrs Maguire then she might still be alive today.
"After it happened I had a nightmare. I felt like I was there seeing him do it. Seeing him stab her.
"I think about it all the time, what I could have done? I imagine telling a teacher about it and then it all being right again.
"I wish I had told a teacher but I just didn't believe he'd do it.
Meanwhile, the family of murdered teacher Ann Maguire have thanked those who've sent them messages and donations.
The money, which has already topped £32,000, is being used to set up an arts education fund in her memory.