Foxconn Technology Group is denying reports in the Chinese media that it forced vocational students to work at factories that make iPhones.

The controversy targeting Apple’s manufacturing partner in China comes as Apple is expected to unveil the latest iPhone on Wednesday.

Foxconn, which in recent months has come under heavy scrutiny in connection with working conditions in its factories, has conceded that it uses students on assembly lines, but says they are not required to work there.

Advocates for workers say the practice of having “interns” work on assembly lines at Foxconn dates back years.

Frequent Foxconn critic Li Qiang of China Labor Watch said the interns are typically 16 to 19 years old and earn about $285 a month, the same as other workers.

Some don’t object to the work, but others feel coerced because they are told by their schools that they can’t get the credits they need to graduate unless they do the internships, Li said.

The schools receive about $100 per student to place an intern at the factories, he said.

Last week, Chinese state-run media reported that several vocational schools in the eastern city of Huai’an required hundreds of students to work on assembly lines at a Foxconn plant.

Foxconn told the New York Times that students make up less than 3 percent of its 1.2-million person workforce in China.

Foxconn told Bloomberg that a recent audit of its three factories in China by the Fair Labor Association found no evidence that any interns were required to work.

It has complained that its labor practices have come under such intense scrutiny only because of its relationship with Apple.

An Apple spokesman declined to comment on the allegations but said Apple’s code of conduct requires that suppliers follow local labor laws when dealing with interns and other workers.