SHANGHAI — As Apple prepares to unveil the latest iPhone this week, the company’s manufacturing partner in China, Foxconn Technology, is coming under renewed criticism over labor practices after reports that vocational students were being compelled to work at plants making iPhones and their components.

Foxconn has acknowledged using student “interns” on manufacturing lines, but says they are free to leave at any time. But two worker advocacy groups said Monday that they had spoken with students who said they had been forced by their teachers to assemble iPhones at a Foxconn factory in Zhengzhou, in north-central China.

Additionally, last week Chinese state-run news media reported that several vocational schools in the city of Huai’an, in eastern China, required hundreds of students to work on assembly lines at a Foxconn plant to help ease worker shortages. According to one of the articles, Huai’an students were ordered to manufacture cables for Apple’s new iPhone 5, which is expected to be introduced on Wednesday.

“They said they are forced to work by the teachers,” Li Qiang, founder of China Labor Watch, one of the advocacy organizations and a frequent critic of Foxconn’s labor policies, said in an interview on Monday. Mr. Li said his staff had spoken with multiple workers and students who, as recently as Sunday, said that 10 of 87 workers on an iPhone assembly line were students.