I delayed my twins starting school, mainly because one of them was going through extreme separation anxiety (from me rather than his twin!) so they only did a few weeks in reception before starting year 1. They were together for those few weeks and then separated in year 1.
Then in year 4 the school asked if I'd mind if they were put together as they shared the same friends. So they were in the same class for the rest o primary.
At secondary they were put in the same form as there were only 3 of them from the primary school going to that secondary and the policy is to keep them together so they know someone.
They're year 9 now and have chosen almost the same GCSEs - because they like the same sort of thing. They're not in the same classes though, but I don't know if that was a deliberate decision by the school. They still share a lot of friends though and do most things together. But they don't mind being apart.
I don't think keeping them together to begin with will mean they will never be apart. My two slept together till they were 10 but would be horrified by that now.
They share the same friends because they like the same things, and of course have that in common with their friends, not because they can't be separated as twins.
Also I have never dressed them the same (apart from school uniform, and scouts) and I hate hate hate when other people call them "the twins"
You know them best and if you feel they'd be better of together then talk to the school. They shouldn't have a blanket policy of separating twins because every set of twins is different.