The court rejected an application to review the sentence, filed by both Kulayev and the Voice of Beslan.
The Voice of Beslan, a group of relatives of victims of the attack, claimed that that the original 2006 trial had failed to accurately account for the reasons behind the deaths of the majority of the victims.
Kulayev's lawyers said that there were no witnesses who had actually seen their client shoot anyone during the siege.
"The presidium of the Russian Supreme Court considered the appeal of the verdict handed out to Nurpashi Kulayev by the Supreme Court in North Ossetia and left [the verdict] unchanged," the spokesman said.
Kulayev was the only terrorist known to have survived the September 2004 school siege in the southern Russian town of Beslan, which claimed 334 lives.
317 hostages, including 186 children, died as security forces stormed the building on September 3, two days after the siege had begun.
The late Chechen terrorist, Shamil Basayev, later claimed responsibility for the attack. Basayev was subsequently killed in unclear circumstances in July 2006.
The Voice of Beslan group believes that Russian security forces should be held partly responsible for the loss of life in the Beslan school siege, claiming that the death toll could have been much lower had they not opened fire on the school building.
However, a 2005 Russian parliamentary commission investigating the Beslan school siege dismissed allegations that the rescue operation was botched.
