It's unfortunate to say, but it sounds like you're going to have issues either way it goes =[. Personally, it seems like you have two choices. .
1.) Just suck it up and deal with "dressing out," maybe layer up so your chest appears completely flat.
2.) Stand up for yourself, stick your head up, brace yourself/prepare for the worse, and tell your PE teacher what's going on. Tell the teacher may not help at all, but it's worth a shot.
*Also try telling your principle and find out from your counselors if it's even necessary to take PE again. I know at our school only 2 semester credits are required.
Anyways, back to standing up for yourself. I say be persistent in that you'd like to be grouped with the boys in playing, included with the boys, etc. I'd go as far as just walking over to the boys group myself if that's how your gym class runs. You might get some mess from your peers, but like I said, prepare for the worse.
Honestly, there were days that I really just wanted to skip gym all together, but I sucked it up and by the end of the year I got my way and the teacher was just basically like "Screw it, evidently the kid isn't going to stop."
That's just me, I'm rather stubborn, but if I'm going to be treated badly either way, I'd rather go the route that involves me standing up for what's right and should be. I know everyone's circumstances are different though.
Also, I'm not sure what part of Texas you live in, but I would do some serious research and see if your city has any laws that protect people regardless of gender identity. I'd also reach out to some support groups/legal groups in the area.
Good luck, really! High school is hell man, but you can do it