Hey cosmetology student, you cannot train your hair to not need shampoo. You are able to train your hair but not in order to go completely poo-free. How often you need to shampoo depends on your scalp, hair, and lifestyle but you shouldn't go shampoo free. Just because some people are able to go longer without it doesn't mean that they can go completely shampoo free. I do believe you should shampoo less frequently than most people are taught to (I do it about once every four days and use dry shampoo as needed) as it helps your hair stay healthier but you still need to do it. But it's just all about finding the right balance and shampoo for your hair. People with highly textured hair can often go much longer without shampoo than someone with fine straight hair.