The medicine to this headache and the answer to this dilemma can be pictured in two scenarios, it can be directly used or needs planning with school districts.
The first scenario is easy, you or your guardians talk to the P.E teacher or to The principle and reach an agreement to that you wear track suite pants and a long sleeve shirt under or over your T-shirt. This is what most Muslim girls do nowadays, especially with the help of the sport hijab.
The second scenario is after a school or district does not accept the pants and long sleeve. Therefore, it has to be addressed and fought for by the Islamic community at the district. The school districts have to be aware that it is not just one or two Muslim girls, but Thousands and Thousands nationwide that have to follow their Islamic obligations and keep in their minds that the religious duties are more important for these young girls. So they have to reach an agreement for the sake of religious rights That they believe in.
One may ask what is the duty of these girls until things get right?
The answer to that is to excuse yourself from the class or wear what we stated in the first scenario even if they do not accept it.
If the school or school district do not accept your excuses or make a bigger problem you must cancel your gym class.
If it was impossible to cancel it, you must find a new school that allows the hijab dress code.
Some girls may love sports and that is okay in Islam, some may see moving to other schools as a problem, and some may see the hardship of fighting for their rights as a problem.
Yes, it is not easy, no one said it was going to be easy but these are our responsibilities towards Allah (swt). Each Muslim around the world is tested in a way, and the way Muslims in the west are tested is by keeping their grip on Islamic obligations.
Verily, the hard work of and hardship in the eyes of Allah will not be lost, and which eye is more important than the eyes of Allah (swt).
Never will I allow to be lost the work of [any] worker among you, whether male or female(1)
1. 3:195 Quran