When I used to do it, we asked the children in year 6 to write down the names of two others they want to be with. We asked the year 6 teachers if they had recommendations about who to put together/keep separate. We looked at the mix of boys and girls, ability ranges based on information from primaries, SEN information and anything else that was mentioned or appeared relevant.
It took nights and nights after school, trying to arrange everything to everyone’s satisfaction. There will probably be half year bands too and they have to be roughly equal.
It really is a complex and lengthy process. We even had one form group comprised entirely of pupils coming on their own from primary school. I was always willing to consider a move, if it became apparent that there was a clash or problem, but it can often mean changing subject classes too, so it’s not something taken lightly.
For each child that moves, another has to move too and then it’s possible that someone else isn’t happy about it.
Knowing and liking the form tutor is a good start. The form tutor is your first contact if there is a problem, so having a positive relationship before you start is a bonus. If DS is ok with it, please give it a chance.