A group of 153 schoolchildren was taken hostage by IS in May en route to their hometown of Kobane after sitting exams in the Syrian city of Aleppo, according to HRW.
IS released the last 25 hostages in late October.
Interviewed in Turkey where they were given refuge, four boys from the group recounted regular beatings with cables and a hose and being forced to watch videos of IS militants in combat and beheading captives.
The Sunni militants forced the children -- aged 14 to 16 -- to pray five times a day and beat those who tried to escape or did poorly in compulsory religious lessons, the New York-based group said.
"Those who didn't conform to the programme were beaten. They beat us with a green hose or a thick cable with wire running through it. They also beat the soles of our feet..." one boy was quoted as saying.
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"They made us learn verses of the Koran and beat those who didn't manage to learn them. When some boys tried to escape, the treatment got worse and we were all punished and given less food."
IS has captured large parts of Syria and Iraq, declaring an Islamic "caliphate" and committing a litany of atrocities, including beheadings, mass executions, torture and forcing women and children into slavery.
The group has besieged the mainly Kurdish town of Kobane just a few kilometres (miles) from the Turkish border, sparking an exodus of some 200,000 refugees to Turkey.
Those whose families were close to the Kurdish fighters defending Kobane were singled out by the captors, who were from Syria, Jordan, Libya, Tunisia, and Saudi Arabia, a boy said.
"They told them to give them the addresses of their families, cousins, uncles, saying 'When we go to Kobane we will get them and cut them up'."
The boys said they were given no explanation for their release other than that they had completed their religious education.
HRW said the abuse of the children was a "war crime under international humanitarian law" and urged the international community to swiftly take measures "to stem ISIS abuses."