A Surrey Police spokesperson said officers and South East Coast Ambulance Service (Secamb) crews had also attended the scene.
Collingwood College is a co-educational secondary school with about 1,700 students.
A Secamb spokesperson said it was called at about 14:10 GMT and was joined by Kent, Surrey and Sussex Air Ambulance.
“A student was treated at the scene and taken by ambulance to Frimley Park Hospital in a life-threatening condition”, it added.