Shuriken School: Super Ninja

One night, the Principal comes across a Mysterious Ninja in the school corridors. The Ninja gets away from the Principal, but drops his backpack. The Principal finds all sorts of trophies from Katana School inside. He is just getting over the shock of his discovery when the Mysterious Ninja reappears, snatches the bag away from him and vanishes yet again. The next day, the Principal calls an assembly and asks the guilty student to confess. But no one responds. The Principal now threatens to expel whoever did this as soon as he finds out.