While I'm not in the exact situation as you, as my family isn't really dangerous to me, I also am not fond of disclosing my sexuality to my family lol.
Similar to what the first reply said, I used to wear a flag pin on my school backpack since my family (and many people in my general area apparently) doesn't know the ace flag lmao and I also had the excuse of just saying I liked the colors because black and white is usually my color scheme for clothes, anyway. Still, it can be a bit risky, unless you are truly sure that your parents don't know about any flags. Despite it being a popular choice, I do think you could probably use the "I just like the colors" excuse for a rainbow pin, unless your parents are really that viciously LGBT+phobic or something. If there's anyone at school that actually talks about you to your parents (friends, teachers, counselors, etc.), I wouldn't recommend this.
There's some artists on various social media that have made subtle flag backgrounds for your phone/desktop/anything else that uses a background! I don't have any links to specific ones but you can easily search for backgrounds on Google or any other search browser. Once again, this relies heavily on your parents not knowing about any/certain pride flags. However, there's some backgrounds that very, very subtle so I'd say this would be a better option than just outright wearing a flag pin, haha. But speaking of pins, there are subtle flag pins as well! I've seen them on Etsy but that's just because I use Etsy a lot ¯\_(ツ)_/¯
Depending on your local community and school, you could join any LGBT+ clubs! I imagine the members would be very understanding of your situation and try to help you out :o You could say that you joined a random small club if your school has any (film/anime club, writing club, environmental club, or any club that doesn't involve much work) or that a friend forced you to come with them to a club they had (this is really only believable if you're not planning to become an active member of the club lol....but, like I said, I'm sure you can plan something out with the members once they know of your situation). Once again, though, if you think there's anyone outside of your house who might tell your parents about this, I'd be very cautious u_u
Those are the ideas I have off the top of my head as of right now! Remember, though: your safety is #1 priority! Unfortunately, I don't think there's a way to truly express your identity without having some kind of risk attached to it. I wouldn't blame you at all if you decide to keep your identity a secret to everyone :O Just remember to be careful out there, and good luck! ^-^ <3