But you're looking at it the wrong way. You're not being "excluded", you're not being included.
People really seem to struggle with this. There's no automatic right to inclusion, for you or your DC. People have the right to choose their own friends. Quite often when people complain about the "bitchy, cliquey school gate mums" what they're actually talking about is women who have already founded their own friendship group, often predating the school network. That is their right. It may make you uncomfortable or anxious but that's showbiz.
If you put yourself in their shoes and you had two close friends whose kids were also at your kid's school, you would feel resentful if someone was trying to inveigle their way into your group, particularly, as in the OP's case, because she appears to have identified the "cool group" and shunned the less cool group. It works both ways.
People are open and receptive to other people who have a clear sense of self, who know what they want and plough their own furrow. No one wants someone to socially engineer their way into a friendship group. They'd rather choose people organically, based on who they actually like, as opposed to who they feel they should include.
And the reason this all matters is that if you create an atmosphere of entitlement, where you huff off because people don't immediately invite you into their friendship groups, your children are going to bear the brunt of it because they pick up on the sense of paranoia and it impacts their ability to make friends organically.