The co-educational school charges boarding fees of nearly £35,000 a year and former pupils include the Duchess of Cambridge and Samantha Cameron, the wife of Prime Minister David Cameron.
A spokesman said: “This was an inappropriate prank that was not condoned by the school.”
Child psychologist Dr Linda Papadopoulos told the Sun: "Something a 17-year-old might find funny could have a very different impact on a younger child.
”Exposing them to this, particularly in a school environment which should be safe, is not good by any standards.”
