You might find this of interest. I know that it is not exactly what you've asked for, but it is an interesting scheme. It also speaks to Bostonian's comment, that it can be much more challenging to get a "best fit" for gifted kids in middle school (and elementary school).
http://covingtonlatin.org/view/home/about-us.aspx"The School's commitment to acceleration, allows advanced fifth, sixth and seventh grade students the opportunity to begin their high school career. For many of these students, acceleration offers a more challenging and rewarding educational experience than a regular grade process.
Admitted students completing their fifth grade year may enter into the Prep year, which is a combined seventh and eighth grade year. Admitted students completing seventh grade may enter into the Freshman year, also known as Form I. Admitted students completing sixth grade, however, have the option of entering into either the Prep or Freshman year based on a joint determination between CLS and families as to whether students should accelerate one or two years."
