This is exactly why our school district doesn't release class lists until 3-4 days before the new school year starts. We put a lot of thought into class assignments. We try to balance classes as much as possible. We look at academics, social interactions, which parents are high maintenance, ESL students, Special Ed students, GT students, student/teacher personalities, behaviors, leaders, etc. Each grade level works with the next grade level to help create class lists that we sincerely hope will benefit all students in the classroom. Parent requests are taken into consideration, but no promises are made. Class lists are not just random groupings. There is no way that we can make everyone happy. Our goal is to put together classroom communities that are going to be successful communities.
I'm hoping that my 2nd grade daughter will not be in the same class with her best friend. They have literally been in the same classroom since my daughter was 6 weeks old. I want to see her exposed to new people. But, I'm really hoping my Kindergarten son will be in the same classroom with his best friend. With both kids we have talked about how they can still see their best friends before school and at lunch and recess. We have also talked up the opportunities they will have to make new friends and expand their group of friends.
I found out who my son's kindergarten teacher will be when we ran into her at the fair and she let it slip that he would be in her class. Out of four choices, she was my third choice. I'm not real happy about his class assignment, but I won't be asking for a change, because I don't know all of the reasons behind why he was assigned to that class. And, I know my son, and I know that he will do well no matter what class he is in.
Personally, I would not ask for a class change only for social reasons.