No, it is not true that a worker can unilaterally force a company to make them an employee. As I said before, the law determines a worker's status, not the employer nor the employee/contractor. What the SS-8 does is to formally request that the IRS examine the circumstances under which the work was performed, apply the law, and determine the worker's status. If a worker files a form SS-8, an inquiry is then sent to the employer/company and they have the opportunity to describe the circumstances under which the work was performed. Regardless of which party asks for the determination, the other party is afforded the opportunity to respond as well.
From your description, it sounds as if the music instructors would be determined to be independent contractors and the custodial staff would be employees. Length of employment is not a consideration.

