Even though he's afraid, Sasha decides to head back to class and take the banner with him. After all, he's technically just obeying what Nina Petrovna told him to do. Plus, he'll answer for breaking off Stalin's nose when the time comes.
Just as he's coming out of the restroom, Vovka enters and jerks the banner from his hand. He marches about, thrusting it this way and that.
Ooooops. Turns out, Vovka has seen it all: he knows Sasha broke the nose off of the statue, and pulls Stalin's broken nose from out of his pocket.