Just seconds after the school bell sounded and children were crossing the Pitt Town Public School playground to retrieve their lunch from their bags, there was a snap and a loud thud.

A large branch from a 50-year-old gum tree fell and landed on top of three students and a teacher on Friday afternoon at the small school in semi-rural north-western Sydney.

Eight year-old Bridget Wright, a horse lover and high-achieving year 4 student, was pinned beneath a limb of the towering tree and died after being rushed to Westmead Hospital.

A seven year-old boy escaped with minor scratches and his kindergarten-aged sister was admitted with serious injures.