"THE school gates ones always makes me wonder, are they just women talking to their friends? Surely you cant be mates with everyone?"
Possibly some of them are just women talking to their friends, and no you can't be mates with everyone.
But you can be pleasant or polite. If you catch someone's eye you don't have to blank them, you could smile, or nod, or say hello. Especially as you're standing next to them day after day. It doesn't take any effort to give someone a quick nod before you look back to your friends.
If you don't, well fair enough, but there is a difference between vaguely blanking someone because you're chatting to your friends and didn't really notice them and deliberately doing so in a way that excludes them from a whole group just because you've decided you don't like the look of them or something.