By Guy De Launey
BBC News, Phnom Penh
Pich Dy's eyes are welling with tears as she remembers her classroom humiliation.
Pich Dy's father died of Aids, leaving her mother poverty-stricken
"The teachers asked me to stand up," she sniffs. "Sometimes I was ordered to stand for 30 minutes or one hour, just because I didn't have money."
It is something Pich Dy does not have to worry about anymore. At the age of 14, she has dropped out of school and is unlikely to return. Her hope that an education would give her a better future has been dashed.
The story could have come from any poor community in Cambodia. All over the country, teachers' demands for "informal fees" are forcing children to quit classes because their parents cannot afford to pay.
Seng Hong of NGO Education Partnership, an umbrella grouping of Cambodian education organisations, says research shows sending one child to school uses up almost a tenth of the average family income.
"This increases if the family have two or more kids to send to school," he says. "Then they may reconsider which kids should go to high-grade education and which kids should stop."
In many respects, Cambodia is doing remarkably well at school. Registration rates for primary school are high, at about 90%, and in the countryside yellow-washed school buildings are some of the most noticeable landmarks.
Before and after classes, the surrounding roads are packed with white-shirted students, most of them sharing bicycles or walking along the red dirt roads. The numbers dwindle, however, as the students get older.
Cambodia's Ministry of Education has recognised the scale of the problem. The department's Bou Chum Serey has estimated that half of those who start primary school fail to complete their classes.
Poverty is the main factor. The schools are supposed to be free, but in reality they are a major drain on family resources. With about one-third of Cambodians living on less than 50 cents (25p) a day, it can be difficult for parents to justify the expense.
That was certainly the case for Pich Dy's mother, Sophal. She lives with her five children in the community of Chbar Ampov, on the outskirts of Phnom Penh.
During the rainy season the area is flooded, leaving many of the ramshackle houses semi-submerged in murky brown water.
Sophal's husband died of Aids, leaving her HIV-positive and struggling to cope.
Like many others in Chbar Ampov, she gets by on what she can earn from collecting bottles and cans on the streets. It was not enough to cover the "informal fees" for Pich Dy's teachers.
"When my daughter was smaller I had to pay a few cents a day for school," recalls Sophal.
"As she grew up, I had to pay more - almost 25 cents a day. We had no money, so she had to quit and come scavenging with me. I want the authorities, especially teachers, to help children - not take money from them."
The teachers, however, have problems of their own. Before Cambodia's decades-long civil war, they were highly valued and relatively well-paid.
Now teachers live close to the poverty line themselves, earning as little as $30 a month.
"The salary barely covers utility payments like water and electricity. There's nothing left to spend on anything else," claimed one teacher in Phnom Penh, who did not want to be named.
She admitted that she and her colleagues took money from students - for test papers, course materials, or simply for attending class. It was the only way they could earn a living.
Another teacher in nearby Kandal province supplemented his income by driving a motorbike taxi, and saw informal fees simply as a matter of survival.
"Students have many problems, teachers have many problems, people in our communities have many problems," he said.
For the enforced drop-outs in Chbar Ampov, it comes as little comfort to hear that teachers are also struggling to get by.
All 15-year-old Srey Mom knows is that her hopes for the future have been thwarted.
The children of Chbar Ampov play when they should be at school
"I cried when my mum said we had no money to send me to school," she remembers. "With an education, when I grow up I could have a job. All I can do now is make half a dollar from scavenging."
The scene at Chbar Ampov speaks for itself - dozens of school-age children play in the filthy water when they might be expected to be in class. The situation is similar in other parts of the country.
Everyone seems to agree that the best way to change the situation is to raise teachers' pay, but there seems to be little chance of that happening quickly.
Cambodia still depends on overseas donations for about half the national budget, and the teachers will continue to rely on their students to top-up their salaries.