Firefighters and paramedics rushed to Shenley Brook End School to rescue an injured man from the first floor of a building at lunchtime.
The man was put onto a stretcher fixed to the top of a turntable ladder and lowered into an ambulance.
He is understood to be part of the construction team working on a new £7 million extension for the secondary school.
The site, which is still under construction, will include a new sports hall, science laboratories, classrooms and an enlarged theatre.
The rescue did not disrupt the school day and the man is now being treated at Milton Keynes hospital.
Two appliances and crews from Great Holm and one from Bletchley attended.