Foxconn, which builds gadgets including the iPhone and iPad, said it employed the underage workers as part of an internship programme at a factory in eastern Shandong province.
It issued a statement saying: "This is not only a violation of China's labour law, it is also a violation of Foxconn policy.
"Immediate steps have been taken to return the interns in question to their educational institutions.
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"We have found no evidence of similar violations in any of our other campuses in China, but we will not hesitate to take immediate action in any campus if any violations are discovered."
Foxconn employs 1.2 million people in China, around 3% of whom are interns.
Geoffrey Crothall, a spokesman for China Labour Bulletin, described them as a "cheap and convenient source of labour" that some vocational schools are happy to provide, as it helps boost their revenues.
"The enterprises tend to be factories that need more hands on the production line," he said. "There is no real training or apprenticeship involved here."
The discovery of underage workers is a fresh blow for Foxconn, just weeks after a brawl involving nearly 2,000 employees at one of its plants brought production to a halt.
Earlier this year, the Fair Labour Association found some staff were forced to work more than 60 hours a week , and sometimes for more than 11 days in a row.
In 2010, 13 workers committed suicide amid claims that Foxconn ran a military-style prodution line on which employees were told to work overtime for low wages.
The company denied the claims, but promised to hire more counsellors and set up employee groups to watch for signs of emotional stress among staff.
Earlier this year, Apple chief executive Tim Cook visited Foxconn's Zhengzhou Technology Park , which employs an estimated 120,000 people in the northern province of Hebei.
The company's late founder Steve Jobs once claimed the company was "not a sweatshop" .