I work in a cinema, and even if there is the slightest doubt that you are 15 you will be asked for ID. Or that's how it works at my chain anyway. If you are found trying to get into a film without being the right age, you can be kicked out and banned. Pre-booking the tickets and then printing them off at the machine wont help, as you have to hand your tickets over to someone to check, who will double check the age as well.
If you pre book them and then get turned away, you wont be able to get a refund.