Parents of pupils at King William's College have been told their principal will be absent for a number of weeks.
A letter from the school says Dr Sarah Welch has gone on leave for personal reasons.
The school's senior management team has taken over, which is the normal procedure in such circumstances.
No specific reason has been given and, while confirming the principal's absence, the chairman of governors says he will make no comment.
Dr Welch joined the college just over a year ago as successor to Philip John.