Ok, I can give you some insight. Mind you this is just the paraprofessional talking, but it may help.
First off, our school had an optional International Baccalaureate program, (IB), so the teams were decided as IB and Non IB.
Now the school is completely IB , there are other criteria.
My team is somewhat skewed. We have a large proportianate number of special education (IEP) students on our team. There are certain ratios for classes (can only have certain amount in each inclusion classroom, I think the maximum amount is 9 per class).
I can't say that sex is an issue, because our team is very boy-heavy as well. I think its almost 75-25 in favor of the boys. (Some classes have only 3 girls and 21 boys!).
Yes, I do think test scores had something to do with it as well. I do think learning styles/teaching styles had something to do with it as well.
I do think they try to separate the behavior children as much as possible, but that doesn't always work.
We also have 107 students on our team.