Emergency services were called to report a stabbing at the school site.
Metro UK reported that the Police and the London Ambulance Service rushed to the scene and discovered two 15-year-old boys suffering knife wounds.
Both have since been taken to a south London hospital for treatment.
One teenager remains in a critical condition. The other boy is in a stable condition.
A teenager has been arrested in connection with the incident.
He has been taken to a south London police station.
The investigation is ongoing.
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