I should have posted more fully.
a) you'll get far more responses if you ask about for school sport issues in Primary Education
b) I was agreeing with the idea that this is about the school PE curriculum, and differentiation applies for weak and strong performers across the board.
It's like the school play - it's definitely a school play, even if the performances are just out of school hours. And though you include every child in drama lessons and rehearsals, not ever child will get a starring role.
Now, the younger the pupils, the more inclusive the aim, as every single teacher knows that children mature at different rates.
But just as not every child will be good enough at maths at any given moment to be in the top set, or (under old requirement), good enough in any one thing to access the enrichment programme (no longer mandatory, but not vanished totally either); not every child will be good enough to make a selective sports team.
The crux of supporting a selective school team to to make sure the school puts just as much effort into the non-squad members. So it enriches the PE experience for everyone, and gets the best out of every single child whatever their level. (Like any subject really)
If it's genuinely extra-curricular (ie training in non-school club even if that club has some teachers among its coaches, team is a club team playing against other clubs) then the school won't be able to do a thing about it (other than insist on a name change if confusingly similar) and all you can do us vote with your feet and remove your child from that activity.