Emergency services were called to La Salle Catholic College on Chapel Road, Bankstown, at about 3.15pm yesterday afternoon.
They found a 15-year-old boy with lacerations to his back. He was treated at the scene and taken to hospital in a stable condition.
A second 15-year-old boy was treated for a laceration to the thigh, while a third 15-year-old boy was treated for swelling and bruising to the face.
A number of people fled the scene before police arrived.
Two teen boys, aged 15 and 16, were arrested at a Greenacre home at about 5.30pm.
Another two 16-year-old boys were arrested in Yagoona at 7.20pm.
And a fifth boy, aged 15, was arrested in Bankstown at about 8.30pm.
All five were charged with reckless wounding in company and affray.
They were refused bail and will appear in court later today.
Inquiries are continuing.