In response to Philip Keller's question (below), my interpretation, based upon what is happening extensively in elementary schools in my area, including in my wife's class(who is heavily into this, by coercion) the answer to the question is...
No, you are not already meeting differentiated instruction, but you are now in a position where it actually might be possible to meet the goals of differentiated instruction.
If students are properly placed, the honors section ought to have students that are similar enough in ability that you should be able to get by with the "single approach" of lectures, labs, problem sets, group work, etc.; that is, what we usually do. This is not guaranteed, but if there are special needs students in the honors section, it's probably just one or two students, and they should be relatively bright.
You might also get by with the usual approach in the regular class, because all material is presented at lower level with lower expectations. Thus, if the selection process is well done, most students' needs will be met by the usual lecture, labs, problem sets, group work, etc at this lower level. However, you'll probably have a few more students who are struggling, and you'll need to figure out if they need a special approach (and then use that approach for them) or if the struggles are caused by apathy or otherwise not putting in the required effort (then you'll need to light a fire under them or somehow solve the apathy problem).
The challenged class will be the greatest challenge to teach because they are not likely all suffering from the same problem. So you need to figure out each students needs and try to address those for each student. However, at least now you have a smaller class, and you aren't trying to meet these needs simultaneously (i.e. in the same class) as if the regular students and gifted students were also mixed with these challenged students.
In summary, by ability grouping the students, the range of problems within a single class will be greatly reduced in the top class, somewhat reduced in the middle class, and could still be severe in the bottom class... but now you have three-times the time to work with it, and fewer students each time.
I think those in large schools can often do this. The administrators are probably willing to hire enough teachers because there are going to be multiple sections anyway. The question is whether to ability-group the multiple sections or not.
Many small schools have one physics teacher, who also teaches chemistry, and might also teach general-science or physical-science. Breaking the "physics" class into three levels probably requires an extra teacher, and ability-grouping the physical-science and chemistry classes is definitely going to require extra teachers. The school might not be able to afford that.
In my area, at least a few years ago, the largest school indeed did hire an extra chemistry/physics licensed teacher just to teach the lower sections of the ability-grouped physical-science and chemistry classes. These were mostly inner-city students with a whole host of problems. The teacher was one of my advisees when she was in college. She only lasted three years and burned out in that difficult position. I don't know who her replacement is, and I don't even know for sure if they are still grouping students the same way, but in that setting they just about have to group them. We're talking drugs, weapons, poverty, abusive parents... you name it.
Michael Horton recently said, "I won't even get into the research on tracking." Okay, but it is one way of trying to deal with this. There are clearly those people who feel there is greater good than bad to integrate the best students with the worst students. There are clearly some advantages to that, but also clearly some disadvantages. Choose your poison.
Michael D. Edmiston, Ph.D.
Professor of Chemistry and Physics
Bluffton, OH 45817
From: firstname.lastname@example.org [mailto:email@example.com] On Behalf Of Philip Keller
Sent: Monday, September 28, 2009 8:46 AM
To: Forum for Physics Educators
Subject: Re: [Phys-l] differentiated instruction
My school teaches three levels of first year physics: Honors, Regular and "Principles". These groups correspond to Tomlinson's challenged, average and gifted. Each of these classes has its own curriculum,has different lessons and different labs, takes different tests...in other words is differentiated by content, process and product.
So in my continuing attempt to understand how to apply this in a high school physics class, now I'm asking if earlier posts had it right: by separating our students this way, are we already meeting the goal of differentiating instruction as defined by experts such as Carol Ann Tomlinson?