The male suspects, aged 11 to 18, were detained late Friday to early Saturday close to the boarding school in Kuala Lumpur, said the capital's police chief Amar Singh.
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It is believed the suspects and the boys in the school had a disagreement, and those detained started the fire at the top-floor dormitory early Thursday using gas canisters, he said.
"There was teasing between this group and the religious school students," Singh told a press conference, without giving further details.
He said the suspects' "intention was to cause a fire", adding they could face murder charges.
Six of the seven suspects had tested positive for using marijuana, he said.
The fire -- the country's worst for two decades -- killed 21 schoolboys, aged six to 16, and two teachers.
The victims were left screaming helplessly as the inferno engulfed the dormitory because the only exit was blocked by the blaze and the windows were barred with security grilles.
The dead were buried late Friday after a lengthy process to identify their badly-burnt remains using DNA tests.
Police initially suspected the fire was an accident caused by an electrical short circuit or a mosquito-repelling device but later shifted the focus of their investigation to foul play.
Survivors said they had seen two gas canisters ablaze, blocking the only door to the dormitory.
Singh said the canisters were believed to have been taken from the kitchen to the building's top floor, and a substance was used to make the fire spread quickly.
Police used CCTV to help track down the suspects and they had been remanded in custody, he added.
The blaze focused attention on religious schools in Malaysia, where many Muslims send their children to study the Koran but which are not regulated by education authorities and have faced criticism for being unsafe.
The school involved in Thursday's fire, known as a tahfiz, did not have the necessary operating licences, including a fire safety permit.
About 60 percent of Malaysia's population of more than 30 million are Muslim Malays, and the country is also home to substantial ethnic and religious minorities.
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