It's actually quite simple. Just climbing a ladder. Note that this is a ladder, not a staircase. Try to put a mat under the ladder, or you may slip when you are exhausted.
Prepare a ladder from which the middle pedal can be removed. At first, I climbed up and down repeatedly with a complete ladder. When the limbs feel a little tired during each training, put down the ladder, remove some pedals (according to your actual situation, such as height and leg length), take the opportunity to rest, and then climb the second round. When the limbs feel a little tired again, turn the ladder upside down, take the opportunity to rest your legs while reversing the ladder, and then continue the third round of training. When the limbs are very tired, put down the ladder, remove more pedals and continue to repeat.
At the beginning of training, it is suggested to only carry out the first three rounds, and then gradually increase the rounds.
In the later stage of training, you can try to improve the speed of each round, including the speed of pedal unloading.
Considering that your high school time is tight, this training method can achieve results in a short time, and it won't take too long.
Finally, I wish you success!