Short answer: do isometric exercises.
Long answer, try these (not all are isometric):
Squeeze a stability ball with your feet, and with your legs outstretched (not bent) for 20 seconds, rest 30 seconds, repeat 4 or 5 times. When I say "squeeze", I mean your goal is to try to pop the ball.
Stand with your shoulder to a wall and with your foot resting alongside the wall; using that side of your body, slowly push against the wall with your foot as hard as you can, for 20 seconds. Release, rest 30 seconds, and repeat 4 or 5 more times.
Similarly, stand with your shoulder to a wall; this time, lift your right leg (later, you'll repeat the process with your left) so that your thigh is along the wall and parallel with the floor, your foot will hang down. Next, press on the wall with your knee; you might need to brace yourself on the left using another person or heavy furniture for counterbalance.
Use ankle weights at the heaviest you can tolerate. With your leg straight, lift to the side as high as you can. At your max height, hold for 20 seconds. Slowly lower, rest for 30 seconds, and repeat 4-5 times.
Without the weights, do side kicks in super-slo-mo. Do about 50 side kicks in this manner, being sure to perform each side kick in as close to 20 seconds as possible; hold for 10 seconds; and then reverse the process. Rest 10 seconds in between each kick.
Do the same, but lift in the front.
Do the same, but lift at 10:00 position (45 degrees to your left); then again at 2:00 position (45 degrees to the right).
Do these exercises every other day, do NOT repeat them two days in a row; give yourself another day of rest in the week (so, for example, do them every monday, wednesday, and friday).
Finally, I might add that since you are 15, you might still be growing. It is not a good idea to be doing isometric exercises until after your last growth spurt; if this is your case, reduce the reps and number of sets by about half, and double the number of slo-mo side kicks.