I have a group of friends at school who i hang out with. I'm close to all of them, and especially close to 2 girls, nearly like best friends. But then one day we were on the train together, going home from school. We were all in high spirits, like nothing bad had happened and they were definitely not angry at me in any way. But then the train arrived at one of its stops, and the next thing really shocked me, i don't even know why they did that. They all just got up and left without saying a word to me. Not even a "bye" or anything!
So then i texted my closest friend, who had gone with them, saying "where did you guys go...."
And she replied, "oh we went to Alicia's house" So then for the rest of the train ride, i sat by myself not knowing how to react... Then the next day i saw them, and they greeted me like usual. I don't know why they didn't invite me to Alicia's house, when they had invited EVERYONE, even the girls who weren't that close to Alicia!! I was Alicia's close close friend! Absolutely everyone in my group of friends went...
Then like 2 weeks later, a new girl came to our group. She had transferred from some school... Then they did it again. They all went to Alicia's house without me and they also invited the new girl, who they had just met like 5 hours ago! It's nice that they're trying to make her feel welcome, but even I got pushed aside even though i've known everyone for more than 3 years now and i was close to everyone, but the new girl got invited... Then like a month later, 2 new girls came to our group (Many girls transfer to our school often) and ONCE AGAIN, i got pushed aside and the 2 new girls got invited. Then always the next day after they all have a party in Alicia's house after school, they greet me like usual.
I want to greet them naturally too, and i always smile and its hard to smile when your friends just ditch you for no reason!!
What do i do... why are they doing this?? They even invite the girls they don't like much, just everyone in my group, except me. Even though i am the closest to everyone. I've tried to think positively and maybe its not their fault, but i just can't! Why are they doing this...

That's extremely rude of them! You need to tell them how it hurts you when they do that, and ask them for an explanation. If they're actually good friends, they'll tell you why. Don't blow up at them and fight about it, just act like you're genuinely curious. The calmer you act, the more likely they'll tell the truth, and apologize for hurting you.