Ducky -
Try speaking with the Facilities Supervisor/Head Janitor at your school to see if they have extra rugs or carpets. When my high school mounted a tap show in the gym (oh, the noise!!) we made good friends with all the facilities staff and borrowed a dozen 4x8' rugs - the cheap, rubber-with-fiber ridges type. They were fabulous, and we repaid them with free tickets to the show and a special thanks in the program.
Best of luck!