TWO men in sunglasses and black bomber jackets stopped the 17-year old on the street outside the school.
They knew his name. With a knife, they carved the letters AAA on his chest saying: "You're the first domino to fall. Don't mess with the student fare."
The attack was over an issue that outsiders might find trivial: the price of bus fares. The pupil attacked outside the Mariano Moreno high school in Buenos Aires had been campaigning for them to be cut.
That campaign has succeeded but the tactics of those opposing it have recalled the country's darkest days.
The incident recalls the infamous "Night of the Pencils" of September 1976, when secondary pupils also campaigning for cheaper bus fares were kidnapped, tortured and murdered by the military junta. During the regime's "dirty war" against leftists more than 30,000 people "disappeared".
Hundreds of pupils marched in protest at the attack on the 17-year old. Some subsequently received threats of violence.
Bus fares are a preoccupation in a country where devaluation and the economic crisis has left half the population living in poverty and unemployment running at 30 per cent.
Secondary pupils have been campaigning in the capital for the fare to the be cut to the same rate as in the provinces.
In Buenos Aires primary children pay a bus fare of five centavos (1p) while secondary pupils pay 35 (7p).
In the provinces pupils in the first two years of secondary education pay the cheap 5c rate.
No one can say for certain who is behind the attacks - there are hundreds of bus companies, some operating illegally.
They are attributed to bus "interests", which could mean owners, or unions or both.
The initials "AAA" stand for the para-military "Argentine, Anti-Communist Alliance", a shadowy murder squad operating during the political violence of the 1970s that left an estimated 1,000 people dead and led to the 1976 coup.
However, its tactics have backfired.
A resulting media storm about fares meant that, when pupils returned to school last week, fares in the capital were the same as in the provinces.