A Bay Area university with strikingly high numbers of foreign students receiving U.S. work permits has been slapped with a warning by its accrediting agency and told to produce more detailed information about its students and the employers that hire them.

If Northwestern Polytechnic University in Fremont fails to comply, the school could lose its accreditation, according to the Accrediting Council for Independent Colleges and Schools.

Northwestern Polytechnic has been targeted by critics who have alleged it is a “visa mill” that provides an improper path to U.S. employment — a charge the school has denied. Two years ago, news website Buzzfeed alleged the school used “a system of fake grades” and barred professors from failing students.

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The little-known school ranked first among all colleges of its type in the number of foreign students receiving “optional practical training” work permits, with 11,700 during a 12-year period, according to an analysis released last month by Pew Research. The number of participants in the OPT program has increased 400 percent since 2008, and it is now larger than the controversial H-1B program for highly skilled foreign workers.

Officials at the school, which is located in a nondescript business park in Fremont and was founded in 1984, declined to answer questions about the number of students enrolled or the warning it received last month from the Accrediting Council. ACICS gave the school until June 29 to provide missing information about student demographics and satisfaction levels among students and the employers who hire students and graduates. However, officials did respond to allegations that Northwestern Polytechnic is a visa mill.

“We believe an answer directly from NPU is not as important as the approvals by numerous agencies that authorize NPU to fully operate,” the school said in a statement. “Since the start of negative media about NPU in 2016, NPU has undergone the scrutiny of a number of outside agencies. In all instances, NPU has received appropriate approvals that allow the institution to continue its mission.”

Northwestern Polytechnic did not respond to a request to name those outside agencies.

The type of warning Northwestern Polytechnic received, known as a compliance warning, is typically one of the lowest-level negative actions ACICS can take, said Ben Miller, senior director for post-secondary education at the Center for American Progress, a left-leaning think tank.

“This is a school that two years ago faced really serious allegations, and that didn’t result in getting its accreditation yanked, so it’s hard to look at anything else this agency does with Northwestern Polytechnic and not think it’s an empty threat,” said Miller, referring to allegations raised in Buzzfeed’s year-long investigation.

ACICS president and CEO Michelle Edwards said the agency would “continue to take action, as appropriate, if NPU fails to address concerns” by the deadline next month.

The accreditor has had its own troubles. It was stripped in 2016 of its federal recognition for failing to protect students from fraudulent and under-performing colleges, said Michael Poliakoff, president of the American Council of Trustees and Alumni, a group dedicated to improving quality at colleges and universities. In April, U.S. Secretary of Education Betsy DeVos reinstated ACICS’ federal recognition.

“It comes down very simply to whether ACICS will have the toughness to blow the whistle, and blow the whistle decisively, if — and I stress that ‘if’ — the institution is not living up to the standards of educational integrity,” Poliakoff said.

Most major U.S. colleges and universities are accredited by other organizations than ACICS.

Jay Kumar, a 26-year-old graduate of Northwestern, said his education was “fine” and he had some good professors. But Kumar, who is from India, said his master’s degree in electrical engineering is “not worth anything” because he was denied a two-year STEM extension for his OPT work permit after ACICS lost its federal recognition. Kumar said many other Northwestern Polytechnic grads had similar problems during that time and lost their OPT work permits.

“I don’t want any kind of green card,” Kumar said. “I just want to gain some experience and go back to my country.”

Northwestern Polytechnic is facing a class-action lawsuit alleging it falsely advertised itself as an accredited school when it knew its accreditation was in jeopardy because of the federal action against ACICS.

The suit was filed in Alameda County Superior Court in December by a graduate of the school’s electrical engineering master’s program, Rohit Kalakanti. He is seeking class action status to include bring in anyone enrolled at Northwestern Polytechnic on or after September 12, 2015. The suit pegs the number of students at Northwestern Polytechnic at 6,000.

As a non-profit, Northwestern Polytechnic is required to report financial information, including its revenue, to the IRS. In 2016, Northwestern Polytechnic reported $54 million in revenue after expenses, and said it had $181 million in assets, including $122 million in mutual fund investments, according to IRS data. Peter Hsieh, the school’s president, received $380,000 in compensation.