Charter school supporters and advocates frequently point to the broad choices that families have when seeking a school in New Orleans, where most of the 82 public schools are charters and most accept applications from across the city. But the concept of choice butts against the reality of supply and demand in a city where many schools rate only average or below. Nearly 12,000 children in New Orleans chose their desired schools through the city’s mostly unified enrollment system this spring — but only half got their No. 1 choice, according to recently released results.