A teacher explains to his students in a classroom of a school in Phnom Penh.

Cambodia's annual exams are rife with cheating and corruption, a leading educator said Thursday.

This time of year is exam time, and cheating has become so rampant that police are stationed outside schools to prevent collusion between test-takers and those who might help them.

Teachers, meanwhile, continue to exact tolls on students for passing marks.

Perhaps hard to see from the outside, "underneath the calm [teachers are] selling the answers to exams and taking bribes from students to guarantee they will pass," said Rong Chhun, president of the Cambodian Independent Teachers Association.

As a guest Thursday on "Hello VOA," Rong Chhun called on teachers to refrain from extortion or risk weakening Cambodia's education standards.