How can i keep fit for badminton season?

Im a 15 year old sophmore who made my freshman badminton team last year and i was in the top 10 out of 20 girls. I played #7 singles. I want to keep fit for badminton because over the summer they had open gyms. Unfortunately, i could only go to a few and i discovered that i got worse :(. There arent any open gyms anymore and there wont be until November. How can i keep practicing if the gyms at my school are always occupied and its really hard to play outside?