It is not only pupils who face isolation under Sir Dexter's hard-line regime. Long-serving staff at the International school have complained that they are being shut out.
"We are not being listened to and are being ridden over by Sir Dexter and his lieutenants," said one teacher. "Morale among some teachers is low."
The NUT has called a meeting of staff at the International school on Monday to discuss problems with the leadership of the federation. "Results and good behaviour have come at a heavy price," said one teacher.