Lau Chi-hei, who was 16 years old at the time, had pleaded guilty to arson and possessing items with intent to destroy property.
The court heard that he threw a petrol bomb when several others blocked the road in Mong Kok in February, 2020.
More petrol bombs and “relevant materials” were found in a hotel room Lau and five others rented, the court heard.
In sentencing, Deputy Judge Peter Hui noted materials found in the hotel room were enough to make more Molotov cocktails.
But he said the case happened late at night, and the fire was small and only charred the road.
He said the safety risk was relatively low, and the fire didn’t affect other people or property.
The judge also said the teenager could not have been a mastermind or a key figure in the case because he failed his chemistry and physics at school.
Hui said a custodial sentence was the only suitable option, but added a heavy deterrent sentence was not necessary, because “social unrest won’t happen in Hong Kong anymore as the city has transitioned from chaos to order.”
The teenager will spend between one and six months in the detention centre depending on his conduct.