Do you have to take a pharmacy class to be a tech?

My school offers a pharmacy tech course that I've been thinking about taking. I've been doing a lot of research and everyone says don't pay for a class. I was wondering if I could just by a prep book and then take the exam? Is that allowed or do you have to take a course first? I was also thinking about maybe applying to Walgreens for their training program but I heard retail pharmacy usually overworks you and they don't teach you a lot. Any advice? Oh and also im in TN if that helps

Do you have to take a pharmacy class to be a tech?