M.G.
I have twins, but mine are only two. As a teacher, I feel strongly that I will separate my twins in school. I personally feel it will be good for my boys to be away from each other. It will give them a chance to develop on their own without the other one always by his side. But, many moms choose to keep their twins together. If you are starting to feel the one is following the other, split them up. I'll bet instead of becoming more isolated he will make new friends and talk with new kids. And, if it doesn't work out, you can always put them together the next school year.
Good luck!