Mohammed Umar Khan stabbed Harvey Willgoose twice in the chest with a 13cm serrated hunting knife. Khan brought the knife to school calling it ''scary''. Khan posed with hunting knives and hammer in photos. Khan searched ''zombie killer knife'' online. Khan had prior aggression from inability to control emotions. Psychologist found Khan's IQ in very low average range. Khan weak in language understanding and processing speed. Weaknesses likely from negative early experience. Khan's developmental age was 14 despite being 15. Incident started with Khan and Willgoose squaring up in class. They agreed to take fight outside. Khan told teacher post-stabbing ''I'm not right in the head''. Khan blamed mum for not looking after him right. Khan showed remorse and good behavior in detention. Sentenced to life detention minimum 16 years. Moves to adult prison at 18. Anonymity order lifted to name Khan publicly. Family of victim devastated with grandad dying from grief. Mother calls for knife arches in all schools. Deputy mayor blames societal knife crime roots.