Apple supplier Foxconn has been illegally employing students to work overtime in order to assemble the iPhone X, the Financial Times reported Tuesday.

Six high school students told the Financial Times they would usually work 11-hour shifts to help assemble Apple’s flagship phone at a factory in Zhengzhou, China. The long working hours breach domestic laws regarding student interns.

The report said the six students interviewed were among a group of 3,000 students from Zhengzhou Urban Rail Transit School. The high school pupils said they had been sent to work at a local factory run by Taiwan-based supplier Hon Hai Precision Industry – better known as Foxconn – in September.

Foxconn and Apple were not immediately available for comment when contacted by CNBC Tuesday. Apple and Foxconn acknowledged to the FT that they had discovered cases of student interns working overtime and said they were taking remedial action. Both companies said the students were working voluntarily, according to the FT. The school declined to comment when contacted by the Financial Times.

Read the full article at the Financial Times.