I think at the school gates there is often an element of parents feeling intimidated other parents, so they circle around those who dress similar, speak in the same way, look the same, act the same. I think this applies whether it's because you are wearing a headscarf, have tattoes, dress formally, etc.
At my school it is very cliquey, I sort of presumed it was like that in most schools. There are a lot of Forces children and those parents tend to stick with each other. There are some very local parents and again they stick with each other. There are the 'working' parents who don't really speak to anyone as they are always in a rush!
DD is now in yr2, and there was one mum who didn't even know I was DDs mum til Yr1! I found it very hard in the first few months/year as we live out of the 'local' area, and we both work so we alternated the school runs. DD got invited to a few parties as they were 'whole school' parties, but I am shy, and found it all quite intimidating, so I didn't put myself into group discussions as I didn't know if they knew each other, wanted to talk, was I intruding etc. So, they sort of presumed I was not interested and ignored me back. Also, I am 'professional' so to speak, and when I strode in, in a rush with my suit on, but also with my lipstud in, some parents felt wary of me I felt. I came across as aloof.
I am saying this, as I think it is quite common to feel this way, and it WILL get better and easier, it takes time and effort. The thought of having someone I didn't know around my pokey little flat judging me and having to accommodate the children who I didn't know terrified me, but I did it and slowly I got to know some of the parents.
I am not best chums with any of the mums, but I am comfortable enough to chat to them all now, and I am also comfortable not talking to any of them if I am feeling 'introvert' and they know that's ok. DD has made friends, still doesn't get invited to loads of parties, but, know I am thankful for that as it is tedious going to loads of them!
We rarely do 'playdates' now as I have not been able to do it due to ill health. DD goes on them every couple of months, suits us both fine as we have children in our social circle that we see regularly.