Why don't people say hi back when I say hi?

The last part tells a bit why she might not like you. As for your other classmates, I don't know why they ignore you. But if you're getting the same treatment from everybody, I'd take it as a sign that something you're doing isn't working.

If you want to change this, you could ask somebody why they don't want to talk to you and be open to any criticism they have for you.

Communication takes two people. If you've done things (maybe unintentionally) in the past that have made them not want to talk to you anymore, getting them to talk to you again will mean changing how you come off.

Some people wouldn't care enough to change who they are for other people. They either like them or they don't.

You, however, seem to care a little bit. I personally, like to be approachable. That means making myself so that other people feel comfortable talking to me. There is nothing wrong with changing and improving ourselves when needed.

Some people have no manners. You do you girl.

I always felt it was rude if someone doesn't at least say hi back when spoken to

Have you tried 'Hello' & 'Good morning'?