The students, all 15, were younger than the legal age to perform factory work in China, but they told the Los Angeles Times they were put to work on 12-hour shifts doing assembly on Sony PlayStations.
"They knew how old I was, but they didn't say anything," said one of the teens, a student at the Yantai Engineering & Technology College.
The Times said the incident came to light amid growing concerns in China vocational schools were less concerned with teaching youths a trade than they were in providing cheap labor to manufacturers.
"The vocational schools make money by sending kids to factories," said Geoffrey Crothall, a spokesman for China Labour Bulletin, a Hong Kong worker's rights advocacy group. "It's a fairly manipulated form of labor available to manufacturers whenever they need it."
The Times said Foxconn had issued a statement earlier this month saying it had stopped using underage workers. The statement came out after Chinese radio reported on the situation.
Sony told the Times it had been assured by Foxconn it was complying with labor laws, but said it would ask Foxconn to investigate the students' claims.