My guess for something suitably strong and easy enough for a highschooler would be wood. Take a really big plank of wood and cut and carve it in the woodworking dep till you get a sword blade, and insert a metal rod into for the handle. It would also still be hard enough to cut through that paper mache ball.
I think you're right. A wooden blade could be spray-painted chrome and pass as a metal blade, and if the wood is light enough even a big sword can be wielded easily.
One thing, however, is that such a sword would have blunt edges... and cutting the giant ball so cleanly with a blunt blade would be exceedingly difficult.
Unless Nikaidou has already mastered the wuxia technique of infusing qi in her blade, in which case she can cut anything with the sword of the spirit and doesn't care anymore about the stuff her weapon is made of. 