Inside, a youth club atmosphere prevails, with pupils and teachers competing at the pool table and playing on the shoot-'em-up computer game Half Life in the IT suite.
But local authorities are queuing up to send excluded pupils to the pound;18,000-a-year south London school because of its consistent success with some of the capital's most disaffected young people.
Based in a converted 19th-century convent, Five Bridges boasts an impressive teacher:student ratio, with 10 staff, including a full-time youth worker, to 40 pupils.
Teachers say the school's small classes and relaxed atmosphere are vital for re-engaging young people who may have been excluded for violent behaviour or spent months on the streets. A typical 14-year-old arrives at the school with the reading age of an eight-year-old, yet two-thirds go on to further education. Nearly all gain extra qualifications such as the Certificate of Literacy in Information Technology, as well as their GCSEs.
Anthony Anderson, aged 15, was excluded from his previous school in Lewisham for fighting, but now plans to train as a carpenter. "When I come here every morning I see a lot of friends - and I mean a lot - who are just hanging around in the streets," he said. "I don't understand why nobody's giving them an education."