I'm going out on a limb, but only because you've already mentioned our age. When you took it back to the shop did a parent go with you? No offence, but some shopkeepers may be take advantage of a 12 yr old on their own.
I've not heard of an LFS refusing to take something off a customer that the customer can't sustain. I would give it back even if they could not refund it. I have a pencil urchin and he eats A TON. He's gotten so big that I will be donating him to the LFS for someone with a bigger tank. He was a hitchhiker on my live rock. Not to mention that they tend to knock things over in your tank.
