The attack in May came only two months after his release from a six-month sentence for indecently assaulting a 10-year-old girl.
Kwantreng, of Kentish Town, north London, was arrested after a tip-off to a local newspaper, but released after a cursory check of his alibi, which was discovered to be flawed four months later.
Judge Coombe criticised the investigation as lax and ordered that a copy of his remarks should go to the police commissioner.
The girl was about to return to France after working in Britain as a teacher and au pair. Kwantreng dragged her into the undergrowth. She was punched, bitten, threatened with death, raped and robbed.
Kwantreng was arrested a second time for another matter and the student picked him out at an identification parade. Impressions taken of his teeth also matched bite marks on her neck.