Private tutoring for primary and middle school students has become a huge, financially lucrative market, a survey shows.

Conducted by the Chinese Society of Education and released last month, the survey reveals that private tutoring revenues reached 800 billion yuan ($117 billion) in 2016, covering almost 140 million students. An estimated 7 million teachers, employed by the agencies, serve the students.

About 40 percent of the teachers surveyed teach math at the agencies, while those teaching English and Chinese separately account for 20 percent. Eight out of 10 teachers surveyed hold a bachelor's degree and 60 percent studied normal or education-related majors at college.

While most of the teachers are full-time employees of the agencies, five percent are still college students and 16 percent are teachers at other public or private schools.

With the emergence of the market, a few select companies focusing on private tutoring have also grown into industry giants.

Among them is New Oriental Education & Technology Group, which has provided tutoring services to more than 20 million students since it was established in 1993. Another is TAL Education Group, a leading K-12 tutoring service, which evolved from tutoring solely science-based subjects. Both companies are listed on the New York Stock Exchange in the United States.