I really like using demarara, it's a bit like granulated sugar, seems to come in white and golden. I bring bags of both types back with me whenever we go to England.
I think your son could substitute American light brown sugar in the recipe, unless the teacher is insisting upon demarara, in which case, she might know where to find it over here.
I don't know why we in America don't have the many sugar types that are produced in England. Same goes for cream and milk products: double cream, Jersey cream, clotted cream--yum!