Let me explain what I mean by "student transfers." Student transfers are students who are transferred from one class and teacher to another class and teacher within a school year. Transfers can be made for a variety of reasons, but generally speaking, at my former school they are made for students who don't get along with a teacher. Parents comes into the office and scream at administrators, threatening them with a lawsuit if they don't switch their son/daughter's teacher. Administrators appease the parents and make the transfer. 

For example, this past school year, I had roughly 175 students, about 35 students per class period. Another teacher with the same exact teaching schedule had around 125 students, about 25 students per class period. To an outsider, a 10-student difference per class may not seem like a big deal, but when you're teaching classes such as algebra and geometry, 10 students makes a huge difference. An additional 10 students requires more focus on behavior and classroom management as opposed to teaching the material, thus leading to a reduction in the quality of instruction. Additionally, interrupting a students schedule and flow with one teacher causes the student to lose focus on the material and, instead, increases focus on the policies and procedures in the classroom.

Last school year, I had a student who started in one geometry teacher's class, but because his mom was upset about a pencil-only rule in that class, the student was transferred into my class, requiring his entire schedule to be rearranged. This student's sibling went through four algebra 2 teachers in one school year for similar reasons. Both kids ended up doing pretty poorly in math since they didn't receive consistent instruction.

If that observation isn't clear enough, let's take a look at my former school's API scores. During my first year of teaching (2006/2007), there were far fewer student transfers. Last school year it started getting out of hand. For the 2006/2007 school year, the API score was 721. For the 2007/2008 school year, the API score dropped to 688 — a tremendous drop in achievement. I am a little scared to learn the API score for the 2008/2009 school year, as the amount of student transfers has increased tremendously over the three years.