My dad has always been really supportive of gay rights (he's close with my uncles who are happily in their fourteenth year relationship, one of his best friends is a trans woman, and he was really supportive when I said I wasn't straight). I haven't even told my mom I have a girlfriend yet, let alone that I'm not her daughter. I haven't actually told anyone other than with a few words about being a girl sucking or not liking my body, but they've always been taken as me just hating periods or being an average teenage girl who thought she was overweight. I don't know how to bring it up to my friends, I mean two of them being trans they might take it in stride, but it seems impossible to tell my parents that I want them to use they/them pronouns or that just because I want a flat chest and to baggy jeans doesn't mean I'm a guy. Over the summer I used to think that I could wait the two years until college and start as a new person, so I'd never have to tell them. But after joining the GSA at school I see these trans kids would get to express themselves while I was stuck wearing my old band shirts because they kind of hid my chest one day and severally overcompensating the next with a dress and makeup and jewelry so no one would suspect, just made it worse. I don't know how it say it, or even if I should.
If you're really scared or nervous about telling people, that's completely normal and I think we all go through that to some extent. I know I was terrified to tell literally anyone for soooo long. I think your friends and the people at your GSA would be super supportive if you told them, but still I know it's hard to tell people. You could try writing a letter to them, and have them read it? That way you already thought out exactly what you want to say beforehand. You'd also be able to leave the room if you have to while they read it in case you get really nervous or anxious. Or maybe you could sort of bring it up in conversation, maybe ask some the trans people in your GSA about their experiences, how they knew, etc., and maybe just talking about the whole subject in general will help you ease in to letting them know your own situation. Trust me, once you get through the first couple of people, coming out gets a lot easier for you (that's not to say it IS easy, it's still super scary, but it's not AS scary, at least for me).
I'm still in the process of coming out and it has been nerve wracking to say the least. I've had all positive reactions apart from my mum, which took me by surprise really but she hasn't disowned me. She had some extreme reactions, like anger (which I don't quite understand) but she's got to get used to me not being her daughter. Her choice of language pissed me off too, but I chalk that up to ignorance. Only you can know if you're ready for everyone to know or at least, strong enough to withstand any backlash. Maybe the fact that you're still in school has both merits and downsides but either way, it's your decision to shoulder. Make sure you're doing it for the right outcome too but again, I can't really impart much wisdom on that due to me being quite a bit older.
