I understand your dilemma. I can see you are unsure how your friends would react if you talk to them about what is bothering you. Fact is you should not care about how they will react because as far as you know, they might be completely cool about what you tell them. Vincoid's approach will lead you to finding out if they're real friends or not. Perhaps that is a route you want to consider. On the other hand, maybe you don't care if they're real friends or not-you just want them around as social proof or to strengthen your social life. It all depends on what YOU want.
Instead of doing group invites to see movies, try inviting 1 person. It works better because you are no longer trying to find a time that works for everyone but instead finding a time that works best for you and your friend. You say let's go see Iron Man 3, Tuesday at 7 PM. Now if your friend flakes on you twice; ie. he says he's too busy and doesn't provide an alternative date or time, it's time to say screw him as he's clearly not interested in being seen with you away from school. You move on to the next person and try to do something with them. Your invitations will get rejected-everyone has experienced that-but you got to keep your head high and be persistent.
If you want to be invited to parties, you have to know about them. There are multiple ways to do this. You can ask a friend if they know of any good parties happening.
You: Hey, know any good parties happening this weekend?
Friend: Yeah, one's happening Saturday at this girl's house
You: That's chill. You going to it?
You: Sounds like it could be fun. Mind if I join ya?
Friend: Sure. Bring your own booze though
[conversation continues, you get the address to the house, where you two will meet up, etc.]
You can apply this example to the group of friends you mentioned earlier. By going to parties or doing stuff with them away from school, you become part of the group. But be yourself and do talk with people. I have a friend that whenever I went to parties with him, he would sit in the corner of the room and keep to himself; he spoke to no one and no one spoke to him. Then he would say the party sucked. Guess what? People would not invite him any more. :P
Hope this helps you!