Police in Pakistan's remote Northern Areas said on Friday that a ninth school in five days had been attacked and destroyed.
Local officials have blamed hardline Islamists opposed to female education.
Eight of the schools were for girls, although the latest - burned down in a village near the town of Chilas on Thursday - was a boys' school.
Three people have been arrested, taking the total detained over the spate of attacks to 20.
The schools attacked were mostly set up by non-governmental organisations with foreign assistance.
The BBC's Haroon Rashid in Peshawar says observers view the attacks as a setback to efforts to promote literacy in the under-developed region.
Thursday night's attack was on a two-room community school in a remote village called Akhrot, near Chilas, 120km south of the regional capital of Gilgit.
Police said unidentified people torched the school, destroying the furniture and wooden parts of the building. No one was injured.
On 15 February seven girls' schools under the government's Social Action Programme were destroyed in the Daarayle Valley.
On 19 February a primary school in Chilas was dynamited.
Some local officials blame people opposed to the education of girls.
However, others believe the latest incident shows a more general targeting of international aid agencies by people who regard the construction of community schools with their funding as un-Islamic.
Last year attacks at the offices of the International Fund for Agriculture Development (Ifad) and the United Nations Development Programme (UNDP) in Chilas caused severe damage.
Local officials say they have formed committees to investigate the matter.
A senior government official in Gilgit told the Reuters news agency: "We have about 100 community schools and the attacks have not stopped girls from going to them."
The Northern Areas have a population of around 1.5 million.
The literacy rate is among the lowest in the country at 12% but efforts by aid agencies to raise it have been met with suspicion by some hardline Islamists.
This week's attacks came shortly after President Pervez Musharraf appealed to Muslim religious leaders to help curb extremism.