"Targeted attacks on education are robbing a generation of the chance to realise their potential, with a huge long-term social cost," said Diya Nijhowne, director of the Global Coalition to Protect Education from Attack (GCPEA).
She spoke to welcome a declaration by 37 governments on preventing schools and universities from becoming battlegrounds.
The Norwegian government, which hosted a conference in Oslo on the issue, estimates that 28 million children around the world are unable to attend school because of armed conflicts.
Schools and colleges have been used by armed forces in at least 26 countries, according to GCPEA, a consortium of education and humanitarian organisations pushing for greater protection of students and teachers.
Governments signing up to the Safe Schools Declaration, including several Middle Eastern and South American states, agreed to ensure on a non-legally binding basis that schools and colleges remain off-limits to military forces in a conflict, even when their buildings have been abandoned.
The signers included Nigeria, where Boko Haram insurgents have attacked schools and abducted 276 girls in the remote northeastern town of Chibok in April 2014.
At his swearing in Friday, Nigeria's President Muhammadu Buhari said all efforts would be made to rescue the girls and hundreds of other hostages held by the militants.
Also among the adherents was Afghanistan, where the Taliban have violently opposed education for girls.
Pakistan sentenced 10 men last month to life imprisonment for attempting to kill Nobel-prize winning school-rights activist Malala Yousafzai in 2012.
Yet the country which witnessed one of the Taliban's worst school massacres in the northern town of Peshawar in December 2014 killing 153, mostly children was not one of the signatories to the declaration.
Other countries which did not sign included the US, Britain, France, Russia and China.