Apple has launched an investigation into its supply chain after a workers’ rights group alleged that one of its suppliers was illegally employing students in China to make Apple Watches.

The Silicon Valley giant opened the probe last week, after Sacom, a Hong Kong-based human rights group, alleged that Quanta Computer, a Taiwanese Apple supplier, has been illegally employing students to assemble Apple Watches in the Chinese city of Chongqing.

Sacom said it had interviewed 28 high school students at the Quanta Computer factory in Chongqing this summer. The students said they were sent to the factory by their teachers for “internships”, but performed the same jobs as other assembly line workers and often worked overtime and night shifts, both of which are illegal for student interns under Chinese law.