I use the term "magnet" but it's actually a school within a school...everyone in it is scoring in the top couple percentiles for ability/achievement. Basically what happens is that the district uses a school that has a lot of extra space and designates certain classrooms. I think it actually benefits the district in many ways.
1) they can use the gifted kids' scores to boost up the otherwise pathetic scores of the school on standardized tests. Approx. 20 percent of the school is highly gifted so it helps!
2) It saves on transportation costs because they don't bus the gifted kids from other parts of the district..parents have to drive. Otherwise most of these kids would be bused to their local schools
3) It solves the problem of the building having too much space and not enough kids otherwise.

It got started because parents petitioned the school board, probably citing these various advantages for the district.