Since you're in high school, you don't have much to worry about. You're not going to be the only guy who looks and sounds young. Some people tend to start later than the rest. Leonardo DiCaprio's voice didn't deepen until after he was 16. My own boyfriend didn't begin puberty until after he was 14. Sometimes genetics just happen, and that's what most people assume. Despite what we see in the internet (cuz I swear, we're all over), ->-bleeped-<- is still quite rare. So yeah, most people who see a younger looking guy aren't going to automatically assume "omg, it's a chick!" They probably won't give it a second though.
If you want an example to make you feel better, I'll give you mine. I'm a 21-year-old man, pre-everything, and suffer from chronic baby face. I'm serious, I don't look 21 as either gender XP. It got worse when I cut my hair @_@. My poor boyfriend, everyone thinks he's a pedophile xD. I still go inside the men's restroom, despite my horribly girly face. I don't even have an STP either, I just sit down on the toilet and do my business. If someone asks me what I was just doing in the stall, I'll just give them a horrified look like they're some kind of pervert. It's a justified look too, why the hell would anyone want to know what I just did in the bathroom stall? You likely won't get that though, most men keep to themselves in the crapper.