High-school students hired to assemble iPhone X devices by Apple manufacturing partner Foxconn — rushing to meet holiday demand for the new smartphone — illegally worked overtime hours at a plant in China, Apple has acknowledged.

According to a report Tuesday by the Financial Times, at least six students from a group of 3,000 from a vocational school sent to work at the iPhone X factory in Zhengzhou, China, said they worked 11-hour days, in violation of local labor laws barring students from working more than 40 hours per week.

In a statement, Apple said, “During the course of a recent audit, we discovered instances of student interns working overtime at a supplier facility in China. We’ve confirmed the students worked voluntarily, were compensated and provided benefits, but they should not have been allowed to work overtime.”

The tech company also said, “Apple is dedicated to ensuring everyone in our supply chain is treated with the dignity and respect they deserve. We know our work is never done and we’ll continue to do all we can to make a positive impact and protect workers in our supply chain.”

The iPhone is Apple’s most critical product line, representing 55% of revenue for the quarter ended Sept. 30. The company beat Wall Street forecasts for the period thanks to a bump in iPhone sales.

Apple said demand for the iPhone X, which began shipping this month, was “off the charts” but according to several reports it was forced to cut the expected number of units produced in 2017 by as much as half. The iPhone X — priced starting at $1,000, it’s Apple’s most expensive smartphone ever — has a nearly full edge-to-edge 5.8-inch screen, an advanced camera system, and a new facial-recognition feature used to unlock the device.

“We couldn’t be more excited as we begin to deliver our vision for the future with this stunning device,” Apple CEO Tim Cook said about the iPhone X (with the “X” pronounced “ten”) on the company’s quarterly earnings call.

Apple and its suppliers have come under fire multiple times in past years for mistreatment of workers at Chinese manufacturing facilities, with reports of overcrowded factory dorms and excessive labor hours.

In 2010, Foxconn experienced a wave of suicides and suicide attempts among its workforce, and in 2011 an explosion at Foxconn’s iPad plant in Chengdu, China, killed four people. In 2014, a BBC investigative report found Apple’s standards for protecting workers at Chinese facilities were being violated.

Pictured above: An ad for the iPhone X in Beijing, where the smartphone went on sale Nov. 3.