Wow, I could write two pages on this subject but in the interest of time

I'll keep it brief. I've written about it before on this thread.
I agree that the decision on whether or not to separate should be up to the parents. My b/g twins went to private preschool for 3 years (gotta love those October birthdays, LOL) and there was only one classroom. Teachers there said they saw no problems in keeping them together as we looked toward Kgarten. One of my best friends is a 1st grade teacher and she was putting a lot of pressure on me to separate them. I felt she had old school thinking about it for whatever reason, and didn't fully understand the twin bond issues. Another friend of mine, also a teacher, had indentical boys herself in the same grade as mine, and she was going to keep them together for Kgarten. She happened to be very involved in our local Mothers of Twins Club, and she told the principle at the time there were recent studies showing no harm to twins who were not separated

(or something to that effect). She also asked whether they would require separation for two kids who were best friends but not related. No, right? So why then would they separate twins automatically? These were her thoughts and I thought they were good ones. All I knew was that my gut was telling me no way.
Two weeks into Kgarten I found a lump in my breast and by October I was diagnosed with breast cancer. I was so glad they were together! The support we received from their teacher that year was incredible, she had a young sister who was a long term BC survivor so that was one of the earliest stories that kept me sane that year. I found it was also nice because we knew everyone in the classroom, boys and girls families, and everyone that knew what was going on wanted to help. It was nice just knowing we had support. We went on to become good friends with many families from that period. By the end of that year I hadn't even finished treatment. I cried so hard saying goodbye to their teacher, I felt as if she was one of my many blessings that year.
For first grade, my gut told me once again to keep them together. No problems had been identified with them being together, and altho my DD was ahead a bit academically, DS was not far behind, and I felt there was kind of a "healthy competition" between them. DD's strong suit was reading, and DS's was math, so together they sort of each had something they were good at and they would see the other being good at it and a) become interested in it too and b)
want to become good at it too. Teachers were great and supportive of our decision, and at the end of 1st grade said if we wanted to keep them together, they saw no problems with it.
Second grade came along, and they happened to get a teacher who "never had twins in her classroom before" (I took that to mean she didn't really like the idea of twins together since she is not too far from retirement and no doubt has taught hundreds of children over the years). I wanted to make sure my health continued to be good (which it has, thank God), but if there were problems I knew it would be easier all around if they were in the same classroom. Had there been any problems identified with them beiing in the same classroom, though, we definitely would have separated. Well toward the end of the year at conference I told this teacher we should discuss separation, which was my plan for third grade. Well I nearly passed out when she said to me "Separate? Oh no, you
can't separate them. They're
wonderful together"!!
So here we are in 3rd grade and together once again. Met with the teacher at Open House and asked her how it's going and she said their being together is a "non-issue" which I was happy to hear. They have just about evened out academically, which is not suprising since I've heard this usually happens around third grade. We are going to take it year by year and see what happens. I will ask this teacher her opinion when we're planning for next year, and watch them as the year goes on. They themselves like being in the same classroom, because we of course ask them as well each year as we're making our decisions. Last year the teacher told me that one day DD was sick and went to the nurse's office. She saw my DS getting kind of fidgety. He came up to her desk (he's kind of shy) and asked her if he could go check on his sister, which he did. She thought it was sweet, and also later told me it was fun watching them together, as her father had been an identical twin himself.
That's my two cents. I know what works for one family won't necessarily work for another. Each set of twins has to be viewed on their own, both academically and socially. I just want to say that if people are leaning toward keeping their twins together, if you feel in your gut it's the right thing, then go with it. I think you'll know if it's not the right thing, and if separation is in their best interest. My friend with the identical boys ended up separating hers in second grade which she felt was in their best interest because they were with eachother exclusively and were becoming known as "one". They did fine as well.