The girl the man reportedly assaulted managed to escape and alert a teacher of his presence, but he had already fled from the school grounds.
Headmistress Jenny Burrett said the school's focus had been on the welfare of the girl involved, but added that the school had 'obviously increased vigilance'.
Essex Police said the man was about 30, 5ft 10in, white, with crooked teeth and dark brown gelled hair.
A police spokesman said they couldn't predict the attacker's 'future intentions'.
Felsted was founded in the 16th century and charges up to £26,385 for boarders in its senior department.