Here in the U.S., very few public high schools have single-track education, at least not for "core" subjects such as English, math, and science. There are typically remedial, standard, honors, and AP classes for these subjects. And I would think that if you're OK with multi-track education, it would seem like having a separate school for gifted students is just an extension of that. That said, in my own experience, a…
I think this is a big leap. I went to a large public high school (~2500 students total) with 3 tracks for each subject. I was in the highest track the whole time, but I would not support physically segregating students like me into a separate school. There are a number of benefits to keeping people in the same school:
1. It's socially beneficial to have high school friends who are different than you are. Many of my best friends were not in my classes but we hung out at lunch time and in after school activities. I think we learned a lot of intangible things from each other.
2. Parental involvement. Parents of kids who are high achievers are more likely to want to see the school go in the right direction. Having involved parents keeps the whole school administration on their toes and brings the general standard of the school up for everyone. If the high achievers had their own school the other students would have fewer advocates.
When American schools were racially desegregated we saw gains across the board: https://tcf.org/content/facts/the-benefits-of-socioeconomica...
While separating gifted students is not the same as separating by race, I think a lot of lessons can be learned from that previous experience.