Ngidi teaches at Injabulo Higher Primary, a dilapidated school of around 800 pupils. The local lower primary and high schools are next door. Together they serve an area of more than three million mostly poverty-stricken people, so pressure on places is extreme.
"The kids flock here, and there are far too many pupils in each class: about 60 is the average," she says. Ngidi teaches three subjects history, geography and Afrikaans to four classes, so she deals with 240 children a day. "I used to teach home economics as well, but due to lack of facilities we had to close down."
With 240 pupils a day there are few opportunities to offer pupils individual attention. "Slow coaches remain just that."
South Africa's unrealised goal is a pupil:teacher ratio of 40:1 in primary and 35:1 in secondary schools. However, large classes combined with poverty and lack of facilities, means the education on offer is very poor.
"Facilities and equipment are not available. Since we can't do experiments, we have to concentrate on theory and the kids don't learn everything."
In any case, Ngidi adds, neither the attitudes of the community nor the pupils are good. The school is frequently vandalised, with repairs taking ages, and pupils are slack. "Some are willing to learn, but most aren't, largely because they're distracted by poverty.
"Many children come to school not even having had a cup of tea in the morning. Either their parents are too poor to feed them, or they are away working and the children can't find anybody else to give them food. Many of the pupils are seriously malnourished, and in the early summer especially they suffer from skin diseases."
The only good news is that the area is peaceful, and there is less violence in the school than in many others.
Thembi Ngidi is a remarkable woman. Despite her circumstances and huge teaching load, she is the national president of a woman's organisation attached to the Catholic Church. She has also had six children - four boys, who all have degrees, and 17-year-old twin girls. One is at university, the other at a technikon (polytechnic).
But there is nothing unusual about her experiences as a black South African teacher. Hundreds of thousands of other teachers have it just as bad.
However, conditions across the races for the 353,507 teachers of South Africa's nearly 12 million pupils vary wildly, reflecting in schools the reality that the country along with Brazil suffers the highest income disparities in the world.
Much progress has been made equalising teacher salaries, but with salaries tied to qualifications, black teachers are the worst off. Even highly-qualified teachers don't earn much: rarely more than 1,000 Rand (Pounds 136) a month.
While low teacher morale and lack of dedication are a serious problem, they are hardly surprising. At most black schools classes regularly start late or not at all, and attendance is abysmal.
An African teacher in a poverty-stricken rural area may teach on the ground under a tree with no equipment. Ngidi has a room but huge classes, no equipment, very little stationery and few textbooks.
In many black urban schools, teachers work under threat of physical violence from pupils who, because of sub-standard education, keep failing and are in their twenties before they leave. Just under half of all pupils fail their school leaving certificate.
A teacher at a formerly white state school has too many pupils but good facilities and resources. And a teacher at a plush independent school operates in an environment and with tools equal to the best in the world: computers, laboratories, a well stocked library, vast sports grounds.
Apartheid lingers on.