Milwaukee Public Schools administration has always had some trouble dealing with success, and one can understand why. The school system has many schools that are not doing well. When schools don't do well, people often point to schools that do do well and wonder 1.) if the effect can be repeated and 2.) if there's an inequity involved. Whenever an MPS school does well, people want to know the answer.
Here it is, so you can stop asking. Schools for academically talented students do well because the students who are there are academically talented. You can't repeat this effect at another school that does not consist of the same student body, but you can water the school down by adding quotas. As for #2, yes, there is inequity involved. It's an entire school of tracking, keeping those out who would drag the school down.
Magnet schools can do well, too. Same strategy. Parents who care about their kids sign up. Kids who care about school show up. Kids who don't care have to leave. Charter, same. Choice, same. No magic bullet. Most stats when measured for economic background hold up. Kids who make up magnet schools tend NOT to be free/reduced lunch, minority, or troubled -- the opposite of the perception of the district.
So that's the problem. For all of the best schools, 1.) they cannot be repeated (unless the demographic of the district changes) and 2.) they are unequal (because of #1). These high-performing schools are promoted as proof MPS can still succeed, but MPS avoids promoting them TOO much because people will ask the questions.
Eventually, when cuts come along, MPS has to decide if it will allow its best schools to fall apart. If the schools lose enough funding, the parents of those kids will pull them out, and, unlike the ultimate liberal hope, those good parents with their good children will NOT disburse among all the other schools in order to improve the entire district. And, yes, if the magnet schools were all closed down and the same kids actually stayed in the district, all other schools would improve, BUT those parents and those kids WILL leave.