SINGAPORE - A Singaporean tutor has pleaded guilty to helping six Chinese students cheat in school exams in a "highly sophisticated" operation using video calling and skin-colored earphones, court documents seen by Reuters on Wednesday showed.

Private tuition is big business in Singapore with parents paying as much as S$700 ($535) for four-session courses and some tutors have become millionaires from the business.

Singapore's reputation for a good education system attracts students from across Asia and beyond.

Tan Jia Yan, 32, worked for a tuition center that offered money-back guarantees to Chinese students if they failed to pass exams and get a place in a Singapore polytechnic.

Tan pleaded guilty on Monday to 27 charges of cheating.

Together with her colleagues, Tan helped attach "wearable Bluetooth devices" and skin-coloured earphones to the students which were connected to discreetly placed mobile phones when they sat their exams in 2016.

Tan then sat the exams herself as a private candidate and used a camera phone attached to her chest to send video footage of the paper to her colleagues via Facebook's Facetime app .

Her colleagues would then call the students to tell them the answers.

Court documents said the "highly sophisticated" operation ran between Oct. 19 and Oct. 24, 2016, before it was uncovered by an invigilator.

Tan faces up to 3 years in prison or a fine, or both, per charge.

Three of her colleagues, including principal of the Zeus Education Centre, had also been charged, but were contesting the charges, media reported.

The principal allegedly worked with a Chinese associate, who would refer students to him, court documents said.

For each student referred, the associate allegedly received S$8,000 ($6,100) in deposit fees and another S$1,000 ($763) in admission fees.

The court documents said the six students were witnesses in the case. (Reporting by Fathin Ungku Editing by John Geddie and Robert Birsel)