The students and teachers of National High School 6 call it "Bloody Friday". About once a month, dozens -- and sometimes hundreds -- of students spill out of this school in the Indonesian capital's south wielding chains, belts, bamboo sticks and stones.
Glass bottles smash and ricochet off the school's high metal gates as students clash with their rivals from nearby campuses in the brown haze of traffic pollution.
Such fights are not confined to this mega city of at least 11 million. Large scale violence between students forms a backdrop to unrestrained urban growth in many of Asia's burgeoning metropolises.
Just a couple of hundred metres from National High School 6 -- known by its Indonesian abbreviation SMA 6 -- is its arch-rival, SMA 70.
Fighting between the schools was at its worst during the Muslim holy month of Ramadan, when one of the schools celebrated a key anniversary, with students regularly battling as local mosques called the faithful to midday prayers, students and teachers said.
The last Ramadan brawl, in October, left four students seriously wounded and one teacher with a head injury, 17-year-old SMA 6 student Yuswa Reza said.
Fights usually start with students from one school crossing into another school's territory to go "fishing" for a confrontation, said Reza, a slightly awkward but articulate teenager who sports a fringe pasted across his forehead.
"It starts with one school fishing, definitely any school that gets fished will also brawl. This school (SMA 6) is famous for its solidarity," he said.
Once these fights start, they can escalate rapidly, with students forming military-like ranks, the strongest on the outside.
"Normally the police stop us by shooting into the air," Reza said.
In Thailand's cities, school fights usually occur between boys from vocational colleges. The schools, which cater to the poor and lower-middle class, teach around 15 million students.
The gang fighting began in Bangkok in 1975, with warring students squaring off on buses, shopping centres and school grounds, said Tawatchai Thaikeaw, deputy director general of the justice ministry's youth protection department.
The fights are spreading to other cities, Thaikeaw said, as Thailand's traditional Buddhist culture and family structure erode and students seek bonds elsewhere.
In the Philippines, similar forces drive fraternity wars and hazing on campuses that frequently claims the lives of students.
The latest high-profile case came in August, when University of the Philippines student Cris Antony Mendez was killed during initiation for entry into a Greek-letter fraternity. His bruised and battered body was delivered to a hospital, triggering public outrage.
In the central city of Mandaue, 20 fraternity-related shootings were recorded by police between January and August of this year.
Most of those who join fraternities in Philippines schools are middle class students seeking protection. Others are students who come from the provinces and seek out fraternities that group them among ethnic or regional lines.
Jakarta's school brawls can be explained by a combination of the city's dearth of public space and Indonesian culture's strong emphasis on personal pride and collectivity, explained Fawzia Aswin Hadis, a psychologist at the University of Indonesia.
"In the big cities there is no room for youngsters to have a good relationship with other people, to have sport or leisure time. It's just like our psychology experiments, when we experiment with rats. If they are in a small environment, a tight environment they want to be free."
"Indonesia is a collective society and (the students) don't want to fight alone. They become brave if they are in a group," she said.
Jakarta's school fights can be deadly, but elicit surprisingly little concern from the public.
As in Bangkok, technical schools for poorer students are well known for violence. But SMA 6 and SMA 70 are among a number of elite public schools where wealthy parents are rumoured to pay large sums of money -- often illegally -- to secure a placement.
"Despite our reputation for fighting, the number of students that want to study here keeps increasing," said Tati Hayati, the vice principal of SMA 70.
Hayati said SMA 70 students had been killed in past fights, included a boy stabbed to death by students from SMA 46 five years ago.
In China, the latest official figures reported by the Xinhua news agency show 23 students were killed in fights between January 2005 to June 2006.
In May, Wu Jianguo, a 17-year-old boy in a middle school in the southern province of Guangdong, killed two classmates and injured four after being beaten by a group of students, drawing nationwide attention to campus violence.
"Most participants in school gang fights are students, but there are also some young people aged 16 to 18 from the outside. Typically, they used to be school students, too, but they were dismissed by school authorities for violating school rules," said Xu Jiusheng, a professor at China University of Political Science and Law.
"School violence is also a consequence of the process of urbanization. A certain number of minors come from the countryside, and when they start to study in urban schools, they easily get psychological problems," he said.
"The family background is also an important factor. The parents divorce, or they are busy doing business, and they don't have time to take care of their children," Xu said.
Despite the grim picture in much of the region, student violence is not a blight on all of Asia's cramped cities.
Although Hong Kong remains a centre for organised crime in Asia, gangs such as the Triads have not established a strong foothold in schools, preferring to stay away from the classroom, experts said.
While some teens do join such gangs before leaving school, their "big brothers" discourage them from showing off the affiliation for fear of needlessly attracting the police, said Y.K. Chu, an expert in youth gangs at Hong Kong University.
Most of the limited gang violence among youths takes place outside school, Chu added, in poor, remote neighbourhoods away from the wealthy city centre.
The case of Hong Kong may be an exception that proves the rule, however. The Asian Development Bank estimates that 1 billion more people will move into Asia's urban areas over the next quarter of a century, and school fights are already an entrenched phenomenon.
At Jakarta's SMA 70 and SMA 6, staff said they could not remember a time before the brawls.
And with a ballooning population and breakneck development forming a noose around the neighborhood in which the schools sit, they might be hard pressed to imagine a time after the fights, either.