Japan currently has 643 such schools. NHK also conducted a detailed survey on 180 of the schools that were started in the past 5 years.
It found that about a quater of the operators are in the education sector, but others are from a wide range of industries such as real estate, temp-staffing and caregiving.
Japan's government is promoting a policy to increase the number of foreign students in the country to 300,000 by 2020 in order to raise the country's international competitiveness.
It has also eased their immigration procedures. They are now allowed to work part-time for up to 28 hours a week. But some school operators have been skirting the law.
Authorities crack down on questionable schools
Authorities have been cracking down on the operators of Japanese language schools for forcing students from overseas to work illegally long hours.
In November of last year, police arrested a senior official at a school in Tochigi Prefecture for allegedly forcing Vietnamese students to work more than 28 hours a week. He was later found guilty.
During the trial, it was revealed that the official found the students part-time jobs through a temp-staff agency and took about 40 percent of their wages as commission. He also ran the agency.
To cover up the wrongdoing, the official is said to have falsified the students' work records to show their hours as shorter.
Earlier in the year, officials at another school in Fukuoka Prefecture were arrested on suspicion of referring students to multiple part-time jobs and forcing them to work illegally long hours. The officials were found guilty.
Prosecutors say they had students work in more than one location, and capped their work at 28 hours a week at each job.
What are Japanese language schools?
The Justice Ministry gives permission to set up a Japanese language school if applicants meet requirements for class hours and teacher numbers. Companies and individuals can also start these schools.
Non-Japanese students attending the schools need to submit documents proving their Japanese language skills and ability to pay tuition. Immigration authorities will issue them visas if they pass screenings. They can then work part-time for up to 28 hours a week.
Guidelines introduced by the ministry in August call on school operators to make clear the 28-hour weekly cap when soliciting students. They also call on them not to refer students to employers or accept commissions without approval. The ministry plans to cancel permissions for setting up schools if the operators break these rules.
Students from abroad are indispensable to Japanese firms
More and more students from abroad work at firms in Japan. Last year, they accounted for one-fifth of foreign workers, at about 210,000.
The number of working Vietnamese students is rising sharply. In the past 5 years, their numbers have grown 28 fold to about 74,500. Vietnam has overtaken China as the main source of working foreign students in Japan.
Language school staff member: Schools think profit is more important than education
NHK spoke to a Vietnamese worker at a Japanese language school. He says an increasing number of the schools are accepting foreigners whose main objective is to work in Japan. He says this is making language schools business entities that exist to get a labor force and tuition.
He says many students from Vietnam have paid about a million yen, or about 9,000 dollars, to agents operating inside Vietnam as tuition or application fees. He says many students have taken out loans to do so and work multiple jobs after arriving in Japan.
The man says some are taking on night shifts beyond their working hour limit and dozing off during classes.
He says people in Vietnam believe that studying at a language school is the easiest way to get a visa in Japan, and that schools can make a profit if they get enough students who just want to work in Japan. He says for these schools, the most important goal is to collect as much tuition as possible.
To apply for a student visa, applicants need to submit documents to prove their ability to pay tuition. They also must be screened by Japanese immigration authorities.
The Vietnamese staff member says many of the documents are fake. He says that in Vietnam, it's easy to buy forged documents as long as people can afford to pay for them.
He says many Vietnamese workers are farmers at home, but he often comes across documents in which students' annual income is shown as 2 to 3 million yen, or 18,000 to 24,000 dollars--a number quite difficult for an average farmer to earn.
He says the school he works for also turns a blind eye to the situation, even when they are aware that documents are forged. He says they just want to secure the number of students.
New school operators see students as a promising workforce
Officials at one new school say more than one-third of their students are from Vietnam. The school was opened by a bus company that also provides nursing care services.
The principal says the company set up the school since the local community doesn't have enough young workers. The principal hopes that students from abroad will join the workforce in the future.
Officials from a school opened by another nursing care provider say they hope that students from Vietnam and other countries will give a hand at the firm amid a severe labor shortage. They say they want the students to learn Japanese, be certified as caregivers, and join the firm.
Another school started by a construction firm says that nearly two-thirds of its students are Vietnamese. Officials say the business is appealing since Japan is popular among young people in other Asian countries. They say the business has become a new revenue source for the firm, and that they are considering setting up more schools.
Expert: Labor shortage is behind the problem
Kobe University Associate Professor Yoshihisa Saito says Japan is suffering from a labor shortage and wants students from other countries to fill the gap. He says some come to Japan to study, while others do the same to earn money.
He says the number of non-Japanese students is surging likely because they're told that they'll be able to freely work part-time for up to 28 hours a week.
He says many of these students work in jobs and areas unpopular among locals. He says that since Japan is accepting them as part of government policy, officials should work to understand what situations they are facing.