Thea teaches in a Kinyinya school in Rwanda, where up to two million people lost their lives in the 1994 genocide, including half the country's teachers. Some 80 per cent of her pupils are orphans suffering from trauma, yet teachers receive no special training to help them deal with their problems.
"How can you expect children who have lost their relations to share a room peacefully with pupils whose parents took part in the genocide and in some cases actually killed their families?" she says.
Teacher training is virtually non-existent and teaching materials are hard to come by. But Thea says: "I am 100 per cent happy as a teacher because education is the foundation of every occupation. "
Thea's story is just one of many in Portraits in Courage, published by the United Nations' education organisation and Education International, which represents 22 million teachers in 148 countries. The booklet has been produced to mark World Teachers' Day on October 5, an event which aims to draw attention to the importance of teachers and press for improved status and working conditions.
Large numbers of teachers do not receive the training they need to cope with a changing world and the variety of roles now demanded of them, the booklet says.
Many work amid extreme deprivation, armed conflict and daily violence, lacking even the basic tools of their profession.
In 1966 UNESCO and the International Labour Organisation set standards for working hours, salaries, training and career opportunities. Yet teachers in most countries are worse off than they were 30 years ago and their status is lower.
"I've met teachers who regularly go without pay for three or four months yet still turn up to work. Their dedication is amazing," says Dr Winsome Gordon, UNESCO's chief of primary education and co-ordinator of World Teachers' Day.
"Some of them don't have chalk or books. Thirty years ago in developing countries the brightest and best went into teaching." she says.
More than 100 countries recognise World Teachers' Day, launched in 1994, and are planning special events, such as seminars, round-table discussions and TV and radio programmes.
Lorna Davids, also featured in the booklet, teaches in a slum in the centre of Johannesburg, South Africa. "The climate of violence that reigns in the community has an enormous influence on the children. To teach values and undo their strong inclination for violence is a huge challenge," she says.
And mother-of-six Vanpheng Phendalit teaches 33 hours a week in a village primary school in the south-east Asian country of Laos, where almost 40 per cent of primary teachers have no qualifications.
In order to support her family she has to sell vegetables in the market. She had a year's formal training nearly 35 years ago, and she's exceptional. Then in 1995 she attended a 10-day refresher course and realised how much education had changed.