The procedure was carried out in Southampton, Hants, as part of a government initiative to drive down teenage pregnancies.
As many as nine secondary schools in the city are thought to have been involved.
But it has caused a backlash from parents who weren't aware that their daughters had been fitted with the 4cm device, which sits under the skin.
Advertisement - story continues below
It is currently unknown exactly how many youngsters have taken part in the scheme.