Can I enroll in a high school in October/November?

I’ll be immigrating to the US with my family. We will get our visa around late september if things go ok. Can I attend when I get there?

And if I can, will I be in a significant academic disadvantage? I’ve already completed my 11th grade here. I want to do it again to become familiar and increase my college chances.

Yes. You just need to register to attend whatever school you’re going to. In my district, people generally call ahead to schedule a registration appointment, complete the registration, and then the student can attend school the next day. Call your intended school and figure out the specific steps you need to take in that district. I don’t think you’ll be at a significant disadvantage, especially since you already speak English and will be effectively repeating a grade. We’re looking forward to having you here in the US! Good luck!

Yeah, you’ll have to figure out with your teachers how your grade in the class will work for the first semester and catch up if you need to but you should be fine; you’re not the first person to the this and you won’t be the last. I’m going to join @ski_racer in pre-welcoming you, the US is a nation of immigrants and its great to have so many different cultures. If you don’t mind me asking, as I’m genuinely curious, what country are you coming from? Your English is excellent.