Teenage world is cruel. A teenager will invite you to a party if he/she wants your presence. If you're NOT invited to a party, it means the person who's holding the party doesn't think your presence is suitable for parties.
It doesn't mean he/she doesn't value you as a person, but they definitely don't value you as a party person. And that's not a big deal, if you ask me.
I don't know you personally, but maybe you're just too quiet. Or too self conscious.
Listen, the best way to make a conversation is to take it easy. Sometimes just "yeah" or "hey mate" are enough to make the other person comfortable, to show your affection, and to show you got the initiative. And people respond positively to those things.
For example, what do you do when someone from your school scored a touchdown? Admit it - you'll ignore it. You don't care about the effort that that boy made, you don't care about his ability, his talent, you don't care that he practices every day and takes a beating to achieve a goal, you just don't care about it and you don't respect the effort.
Next time just say "good game/pass/run yesterday, man" when you're passing by him. Without any admiration (or he'll think you're gay or something), but show you CARE and RESPECT.
Ask yourself - why should a quarterback care who you really are deep inside? What's in it for him? Especially if you don't care who he is on the outside or the inside? And I am 100% positive you DON'T care.
I was also lost when I was your age. Later in life, I realized I was always a strong man, but just couldn't find my way at that age. Too many distractions and too many doubts.
It's tough. But just take it easy, let it go, it's not a big deal. Just find something to do and open yourself towards other people. They're not that bad.
There's an old saying: "If you lock the front door, bad people will still find their way in. But good people won't."
So, open the door. And start thinking and caring about someone else, not only yourself. It's not that scary.