When I took a sweater class at my LYS, it was $50 which they applied toward the purchase of yarn. I think you could bring your own, but then would just be paying for the class. I thought it would be a better deal to take the $50 credit. I must have been insane because the yarn we were working with was so expensive. I spent about $100 to $150 in addition to the credit. I don't remember the exact amount, I think I blocked it out.
You are smart to think of that ahead of time.
I would call the store and ask. They should tell you if they care or not. Sometimes if you make a "faux pas" like that you could be given the cold shoulder, yet no one would say anything outright. I have seen that happen (in a store I don't frequent) when someone wanted a shop owner to wind a ball of yarn purchased online. I was so uncomfortable watching the conversation! Anyway, you would not want that to be you during class if you brought yarn and they were miffed about it.