Contracts are a touchy subject with many students and instructors, both sometimes claim that such written agreements are needed to protect, while others claim that the martial arts have a tradition of only a verbal contract binding the student and instructor together. If the martial arts school is approached strictly as a business venture, then perhaps the former position makes sense, but at the same time, based on tradition, the latter may be appropriate.
One way to increase your income is to offer equipment for sale, such as tee shirts, uniforms, patches, videos, books, etc. You become a wholesale buyer, which means in most states that you must obtain a vendor's license. You must also have the funds to purchase the initial inventory.