Find a bum. Kill him with the Katana. Drag his corpse to the couch (make sure you don't get blood on anything but the couch or it will show that he was killed and dragged there). Position him so that it looks like you stabbed through him and into the couch. When your parents get home, explain that he broke into your house and attacked you. You stabbed him in self defense and cut the couch. The couch will be the least of their worries.
btw: Don't get so offended that people find the situation funny. Heck, I've ripped my couch too.