i know. it's frustrating. it's pretty hard to convince people they're wrong about what they "know." it's funny though, because to me, the dumbest part of the "humans are omnivores" argument is that biologically-speaking, carnivores and omnivores are prepared to kill the animals they eat with their claws and teeth. we are simply not designed to kill most animals with out bare hands. so our ancestors watched other animals eating and said, "hey--i wonder what that shit tastes like?" and they used their actual biological advantage (their big brain) to create tools that worked like the natural tools carnivores and omnivores had. after killing some shit, they said, "hey--that's pretty disgusting, and it makes me feel so sick!" so they tried cooking it, like they did with grains, and they liked it. but figuring out how to mimic a carnivore/omnivore does not make you a carnivore/omnivore.

maybe you could ask your teacher whether it's natural for humans live on a purely carnivorous diet. because that's the traditional maasai diet. i'm pretty sure she'll tell you that humans are omnivores, and that the maasai diet isn't really healthy. but the thing is, the maasai have survived on that diet for hundreds of years. because they have acclimated to it. but that doesn't mean that humans are carnivores.

my personal theory on why most people find this argument unconvincing is that they have a subconscious defense mechanism that makes them unable to grasp a reality that says what they do themselves is wrong. if you really want to try with someone, though, i think the best argument is to point them to all the studies out there demonstrating that vegetarians and vegans live longer and healthier lives than meat-eaters. (e.g., http://www.huffingtonpost.com/2008/07/13/benefits-of-vegetarianism_n_112431.html, http://www.telegraph.co.uk/health/healthnews/6710896/Vegetarian-low-protein-diet-could-be-key-to-long-life.html) it simply doesn't make sense that people would be healthier and live longer while eating an unnatural diet.