While Carlmont is the indeed the "primary" high school for Belmont, residents can petition to go to any of the other three if they choose. That is how so many San Carlos residents have ended up at Carlmont, despite the fact that they are zoned in Sequoia's boundary.
Hope this helps....
If you are asking if it's possible to get into another H.S., Maybe. But you would need to check with San Mateo school board to find out if they will make exception.