I shared the following article with a principal a couple years ago.
http://www.washingtonpost.com/wp-dyn/content/article/2007/11/03/AR2007110301167.html
He told me that anything that even looked like "tracking" was a non-starter in our school district. Fast forward a couple years with significant budget cuts and higher student to teacher ratios, now our school is experimenting with some ability grouping again. It seems that heterogeneous classes were easier to manage when class sizes were 20-24 kids. Now that class sizes are closer to 30, not so much. Unfortunately, the first attempt was hugely successful for the high end kids and a disaster for the lower end class that ended up with the high energy boys with behavior issues. The teachers fought against a second year of this model saying that it was unfair. The current iteration only does ability grouping for language arts with kids shifting teachers during that block according to reading ability. People seem to be happier with this compromise.