My DS attends a gifted magnet public elementary school which feeds (guaranteed seat) into a public middle school IB program. Some choose to go that route, others go to their regular neighborhood school. To be considered for the IB program, a student must have a certain level on their EOG tests. If they don't meet the criteria then you don't get in...either they go to their neighborhood school or in the "general population" of the IB magnet school.
The two middle school IB programs are highly regarded here. There is a lottery with long wait lists. Both middle schools feed to high school IB programs.
Hope that gives you some more info... DS is in 4th grade, so we have another year to figure it all out
