The principal of the Bright Future High School has decided to have trees planted on a playground where the energetic schoolboys play every day. To protect the young trees, circles of thin iron poles are driven into the ground surrounding the plants. During the first break, Tim, one of the boys in grade twelve, tries to sit on one of these poles. His friends warn him that the pole is too thin and has a too sharp tip, but he nonetheless tries to sit on it. The pole penetrates his buttocks and causes a serious injury.

Tim's parents institute a delictual action against the Bright Future High School for its omission to guard against the poles injuring the schoolboys. If the management of the school concedes that the omission was wrongful, what defence can the school nonetheless raise and what will the effect of a successful reliance on that defence be? Explain in detail