The All Nation's Centre just off Kennington Lane is a hive of activity.
Downstairs, the stage is set for a gospel concert that evening, while upstairs the 60 children of the Lighthouse Saturday school have been hard at work for an hour already.
As the noticeboard testifies, this church-run community centre is well used. The Saturday school started ten years ago after a group of parents resolved to pool their intellectual resources to help children who were struggling at school. Demand soon saw it move onto a much more professional footing.
"When it was really growing, they said 'Hang on a minute, we need to have more of a structure to this'," says Florence Bankole. "So that we can help more children, not just children who come to this church."
Florence is the school's co-ordinator ("I suppose you could call me the headteacher") and a classroom teacher during the week at Charles Dickens primary school in nearby Borough. In fact, all six teachers at Lighthouse are fully qualified, and paid accordingly. But like Tinu Ademefuh, a teacher there for seven years, they are mainly motivated by the social purpose.
"You tend to have extra time to spend with an individual child," she says.
"The classes are smaller and there are fewer behavioural problems - they want to be here. There's a lot of parental support as well."
The school day, from 9am until midday, devotes an hour each to the core subjects of maths, English and science. The atmosphere is calm and relaxed, but there is a tangible sense of purpose. Pupils are enthusiastic - when one teacher announces they will be doing maths next instead of science, a cheer goes up. Discipline is important.
All new pupils have to sign a behaviour contract and risk losing their place if they miss two classes without good reason.
Aged from five to over 16, they are arranged in small classes of less than ten, getting the kind of individual attention that can't be guaranteed in ordinary school at a fraction of the cost of private tuition - parents pay just pound;6.50 a day, supplementing the grants received from bodies such as Trust for London and Children in Need.
Like its mainstream equivalent, Lighthouse has an academic year of 39 weeks. Pupils are given homework and receive regular reports on their progress. Some staff teach children during the week as well, but contact with the children's mainstream school is kept informal.
One thing Lighthouse lacks is a place to call its own. Because the community centre is used during the week for other things, all the school's materials have to be packed away and pupils' work cannot be displayed.
Diana Omololu, a fund-raiser, is looking for new premises nearby to give the weekly school a more permanent feel and to house some of Lighthouse's other educational initiatives such as the after-school online homework club (at a nearby school) and Saturday's optional extras like keyboard lessons or extra tuition for secondary school entrance. Upstairs, secondary class children are learning about adjectival endings.
Teacher: "A hat from Mexico is what?"
Pupil 1: "Mexican."
Pupil 2: "I thought it was a sombrero, Miss".
Next door, Years 8 and 9, preparing for their GCSEs, are dissecting Pythagoras' theorem.
"It's like a second opportunity," says Fola, taking a break from his equations. "You could be doing something and when you get to school you've already done it and then you can help other people. It makes you feel proud of yourself."
Florence Bankole is proud of the way the school helps children like Fola's classmate, Ade, newly arrived from Nigeria and struggling to get used to the culture of mainstream school.
"When they come here they are more relaxed, and it helps them. People say to us how can you prove you are having an impact," she says. "When children do their exams we cannot claim that it is through us that they have passed.
But we have contributed to it. If a school is not contributing to their education, the children will not keep coming back. And if you have children from reception to secondary school age, you must be doing something right."