"Boko Haram's attacks on schools represent a new and reprehensible development since the group began its campaign of violence in 2009," Zama Coursen-Neff, deputy children's rights director at Human Rights Watch, said from Nairobi. "Children and educational institutions should be left alone, full stop."
The rights group said at least 12 schools were destroyed this year in northern Nigeria by the Islamist militant group.
Abul Qaqa, described by Human Rights Watch as the "purported" spokesman for Boko Haram, said the attacks on schools were in response to the arrests of Muslim clerics by members of the Nigerian security forces.
Abuja in December declared a state of emergency in parts of the country because of militant activity attributed to Boko Haram. More than 100 members of the sect were arrested following a January attack on eight government sites in Kano in northern Nigeria.
Boko Haram seeks to establish an Islamic state in Nigeria, a country divided largely along Muslim and Christian lines.