This is absolutely tragic and I cannot comprehend what her family is going through. Many lives blighted, I agree, but what kind of a person must you be, irrespective of your age, to stab a 12-year-old girl to death?
To address Sqad's point, I wouldn't have let my 12-year-old go to town with their mates on a Saturday night either. We would have taken our 12-year-old to see the lights or whatever, as a family, but not sent them out with their friends. I turned 12 only 6 months after leaving primary school!