There is no such source.
The laws of war prohibit direct attacks on civilian objects, like schools. They also prohibit direct attacks against hospitals and medical staff, which are specially protected under IHL. That said, a hospital or school may become a legitimate military target if it contributes to specific military operations of the enemy and if its destruction offers a definite military advantage for the attacking side.
If there is any doubt, they cannot be attacked. Hospitals only lose their protection in certain circumstances - for example if a hospital is being used as a base from which to launch an attack, as a weapons depot, or to hide healthy soldiers/fighters. And there are certain conditions too.
Before a party to a conflict can respond to such acts by attacking, it has to give a warning, with a time limit, and the other party has to have ignored that warning. Some States have endorsed the Safe Schools
Declaration and Guidelines, which aim to reduce the military use of schools.
Frequently asked questions on the rules governing war. international humanitarian law and the law of war.
www.icrc.org