Police were called by the London Ambulance Service to Highgate Road just before 2pm today following reports of a stabbing.
Officers and the ambulance service attended and found a male teenager, believed to be aged 16, suffering from stab wounds.
He has been taken to a north London hospital in a serious but stable condition.
An air ambulance was at the scene.
A teenage boy has been arrested on suspicion of Grevious Bodily Harm and taken to a north London police station for questioning.
Enquiries are ongoing. A crime scene remains in place.
In a statement, William Ellis School said: “There was an incident outside the school in Highgate Road at around 2pm.
“The police were called and are dealing with the situation.
“We can confirm that none of our pupils were involved.”
Anyone with information is asked to call police on 101 or Crimestoppers anonymously on 0800 555 111.
You can also tweet information via @MetCC.
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