Your success will hinge on catching the students at exactly the right moment--when they know they're going to take the classes but have not yet bought a calculator. Look for ways to distribute fliers to students who are in the correct majors but have not yet begun classes. This is going to take some effort to puzzle out. Once you have found a way to get fliers into the hands of the right students, be sure to make an offer they can't refuse. I'm not necessarily talking strictly about overall price. You may need to give them all a way to pay you that makes it easy on them, or offer the first week free, for example. Best of luck!