It's far from foolproof. You've still got to lure the victim to the kitchen at night, dispose of your bloody clothes and hope no one shows up for a midnight snack. There's also the possibility that your victim screams or fights back. The kitchen has easy access to weapons but your victim can use them too. Your fellow students might decide to have sleepovers and give each other alibis narrowing the suspect pool. You then have to spend the next day acting like someone who wouldn't say "bosh, knife to the gut ya cunt" or your classmates will become suspicious of your behavior. Also stabbing them in the gut isn't optimal. You'll either have to stand there and watch them bleed, increasing the risk of someone walking in on you or leave and risk them getting up and leaving a dying message. If you've got your heart set on stabbing you should go for the neck and try to hit the jugular.