In many cases, eliminating the tracking system can actually hurt the lower-level students far more than it helps them. When I was in elementary school, my school did track the classes, and the whole school – including the lowest-level classes – had some of the best test scores in our district. By the time my sister got to the same school six years later, they had stopped tracking, and everyone’s test scores dropped dramatically. What @Jayne describes is absolutely true – students benefit the most from having a curriculum tailored specifically to their needs, and if you put all levels in the same classroom, there’s no good way to make that happen.
I also don’t believe that it’s necessary to eliminate tracking in order to diversify the classes and have different groups exposed to each other. Yes, tracking tends to separate those groups out to some degree, but it’s only necessary in some of the academic classes. My school tracked our reading and math classes, where the differences in ability really affected the curriculum, but our “homeroom” group, who we had non-academics with (music, art, gym, etc.) gave us the chance to be part of a very diverse group, in a setting where it didn’t matter what our academic abilities were.
I’m a firm believer that tracking is the best way to give ALL of the students the education they need – it just takes a little more work on the part of the school, to make sure they also create situations that don’t separate the students into racial and economic groups. The diversity in all of the schools I went to was one of the main reasons I loved going to public school – and I managed to get that experience, even though there was a tracking system all the way through my schooling.