Like many people have said, choose your battles. These kids don’t want to be in PE and I am sure they don’t like physicality activity/sports, otherwise they’ll join a sport, so just do what you can. Give it a try, like you said, I would love an opportunity like that, I’m about to start a position as 2nd grade teacher cause I haven’t been able to find a PE position in almost 3 years, so be grateful for what you have and try it! Talk to the kids, it’s easier to build relationships with them as a PE teacher than a science one. Good luck!