I thought we were friends?

I love how people always tell me to cherish my high school days. On one end, I completely agree, I mean those were the days; but now that I think about it, it's interesting to see how close I am to my friends.
For me, I was the quiet one. I'm really shy, so I hated speaking in class and I was terrible at making new friends. Unless the other person was talkative, it was hard for me to keep up a conversation.
Can't say I've changed much now, I still am really shy, but I've become better at spurting random stuff out when it gets awkward. Unless the other person intimidates me. Then I tend to curl up into a shell and just stay hidden.
Anyway, in high school we were always split into two groups. The "cool" group, and then the rest. The "cool" group mainly consisted of those who went into IB and then dropped, most of them were from the same junior high so they stayed together and they rarely sat with us for lunch. They were also usually wearing all the brand names so we referred to them as the "cool" group.

It's always facebook where you see the events or parties that you weren't invited to. I've definitely lost interest in facebook, but I am a curious person; I stay on to creep.
I always try to get everyone included, even if I didn't like someone, I always invited them.
It mainly started when my friend "S" decided to host some event. However she only invited a limited number of people and they were part of the "cool" group for us in high school. I wasn't included, which I found kind of weird because just a week before "S" and I were planning a surprise party for our friend "J".
But both "J" and I were in the same class, so she asked me online and of course I said that I wasn't invited.
She then decided to tell "S" why I wasn't, and "S" took over a week to invite me to the event. For her to even take time to think to invite me made me think she didn't even want me there in the first place.
Not going to lie when I say that I was a little hurt, yet at the same time I wasn't surprised over her decision. She has always been in the middle of our groups and lately she's been hanging wanting to be part of the "cool" crowd.

Now I've seen pictures of parties where I, and several others of our group, weren't invited to. I won't say that it's because we were not "cool", I highly doubt that was the reason, but they were all of our high school friends and the fact that we weren't invited was a little... lonely? Just the fact that we spent so much time in high school (and even some stuck with us in junior high!), and we weren't included.

I thought we were friends, hey?