Is it permissible to build a school using money donated by people to the mosque, when this money is not an endowment and is not currently being used for the mosque's needs?

It is not permissible to spend money allocated for a mosque on a school, except with the permission of the donors. This is because the fundamental principle is the obligation to observe the purpose for which the owners of the money donated it. The jurists have stipulated the necessity of observing the intention of the donor.