A teenage boy has died and another is seriously injured after they were stabbed outside a school in Sydney's west.

Emergency services received reports of a stabbing near Doonside Technology High School, on Power Street in Doonside, about 3.40pm today.

Before police arrived, two teenage boys had been taken to Blacktown Police Station with stab wounds to their torsos.

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It's understood the pair were driven to the police station. 

One died outside of the police station, while the second, aged 19, was taken to Westmead Hospital in a serious condition.

Police have established two crime scenes and are investigating the incident.

One person has been arrested. 

Roads near both crime scenes have been blocked off.

More to come.