Pre-T, you're more unlikely to pass.
And here's why: when boys are around school age, they mostly have short hair. Very few young teenage boys have longer hair because it makes it harder to tell what gender they are. You will occasionally have kids with longer hair but pre-puberty, it's rare. Don't get me wrong, it does indeed happen, but it's not so common. Once those kids go through puberty and have a more male physique with at least hints of facial hair and overall looking taller and older, they can rock long hair with more confidence and better pass rate, although it always should be noted that even then, many will still get misgendered from behind. Right now, you're like a pre-pubescent boy. You don't have facial hair, a man's voice, perhaps not even tall. If you have long hair now you'll get misgendered the way pre-pubescent kids would if they had long hair.
Does this mean you shouldn't have long hair? Hell no! It's your hair. Do whatever you want! You just need to be sure you're strong enough to be misgendered and be okay with it. That's not something I could personally do but power to anyone who can.
Personally I went with dress and hair to pass over what I ultimately want to have which is longer (but still short, just not clipper short) hair and more grown up, smarter clothes. The clothes I'll change up to smarter now and again but I know that I look like a young teenage boy so wearing jeans with skate shoes and a graphic shirt makes me pass a lot better than say a suit to use a polar opposite. For me, personally, passing is important and I pass 100% myself. I'm even able to wear smarter clothes already without it looking too odd. But would I rock a long hair style right now? Well 1) I never would because I have more typically girly, wavy hair and 2) I just wouldn't pass and I know it!
