Read the OP but not all posts. I can relate to this. My DD has gone to Grammar school. She is with two of her friends from primary, and there are another two in another form at the Grammar. However, her best friend from primary went to a different school, and I expected the friendship to fizzle out after a while, but it did almost straight away. They have exchanged the odd text and phone call but that's it. There was a meet up recently at the local shopping centre with her friends from primary and her best friend from primary was there. The girls spent some time together and my DD said that it was nice, but that her friend spent the whole time talking about her new friends.
At our primary, the kids either go on to a Catholic secondary (in most cases) and you get the few that go to Grammar. Quite a lot of parents have said to me that although their child is capable of the Grammar, they have let the child decide where they want to go, and most have followed their friends. Personally, I don't agree with this. I think 11 is too young to make such an important decision. We very much wanted our DD to go to the Grammar as it's a far better school than the comp in every respect. However, I have seen the 'other side' too re friendships and continuity and so on. Still, I firmly beleive that my DD is in the best place. To be honest, there were quite a few kids in her class at primary that she was good friends with that I'm glad to have separated her from. Even her best friend from primary. I often felt that she wasn't a particularly good influence on my DD. She's very 'old' for 11, into boys and so on, and my DD isn't.
She has a good friend from primary at the Grammar with her, and has made a new best friend there and is happy.
The thing to remember is that even if they all go to the same school, quite often they're split into different forms/streams and friendships can fizzle out anyway, as it's so much more of a 'bigger pond.' It's quite likely that they'll make other friends .