Nicholas Elger, a former student of Winchester College in Hampshire where fees cost £39,912 per year, was described by the judge as “very intelligent”.
The 17-year-old was jailed for four-and-a-half years at Winchester Crown Court on Thursday on two arson charges for two petrol bomb attacks on the M3 in September last year.
Judge Keith Cutler sentenced Elger to the prison term with an extended licence of four-and-a-half years.
“I consider you are a dangerous offender,” he told him.
“There is a significant risk to members of the public of serious harm from you in the future.”
The second firebomb incident caused the motorway to be closed at Winchester for 11 hours, leading to major delays and resulting in £40m damage to the economy.
Prosecutor Tessa Hingston said it was a “massive inconvenience to the public,” adding that the “risk of really serious harm was great”.
Elger made Molotov cocktails using items stolen from a Tesco store.
He threw them from a bridge close to junction 10 of the M3, which is the main route between London and Southampton.
The judge described the defendant, who wore a suit and tie in court, as “very intelligent, possibly the most able chemist the college has produced in recent years”.
He added: “He’s a very bright boy and for some reason he takes to burgling the school, blackmailing the headteacher and making incendiary devices and throwing them off motorway bridges.”
Sky News reported in March that Elger had been detained in a psychiatric hospital after Judge Cutler ordered him to be treated.
Elger wrote that he wanted to kill patients and staff by “strangling and jumping on heads” in a diary during his time in hospital, Judge Cutler said.
Ms Hingston said the schoolboy told medical staff: “I do not regret the incident, I regret not doing them differently and not succeeding in killing.”
Elger was also sentenced for eight burglary and two blackmail charges against top public school Winchester College, which he attended as a boarder, and two charges of theft from a supermarket.
The burglary and blackmail offences cost the college £52,000 in stolen items and damage.
Elger twice demanded payment of £10,000 in the online currency bitcoin to stop carrying out further break-ins.
He previously pleaded guilty in December to two charges of arson recklessly endangering life.
Robert Morris, defending, said doctors had ruled out personality disorder or psychotic illness.
He claimed that Elger’s actions had been caused by “immaturity and being upset by his parents’ divorce, leaving his school and support networks”.
Mr Morris added that the defendant had been a “kind and gentle” child who was “playing up to a bad boy character he had created for himself”.