Okay here is the thing.
I was at school, we had break (in school) and I was sitting and talking with a friend of mine (girl) about she just breaked up with her boyfriend. My girlfriend comes over and say hey, and I said hey and kissed her, and started my conversation with my friend again.
She leaved.. And now she is mad and she is saying that I ignored her? But it wasn't on porpuse, and she is mad now, and I want to apolagize..
So what do I do now? And do I say?
Please answer fast.. thanks

Don't apologize, you were involved in a conversation and she came up to you. You were busy, she needs to be understanding. If you two are still kissing, why the break?

Explain what you have explained here.You acknowledged her you kissed her but you were having a serious conversation and the other person needed your attention at that moment.

If she fails to understand that then you have some communication issues that you need to work out.

Talk to her and let her know that you didn't mean to ignore or dismiss her. You were preoccupied and didn't think about explaining the short greeting. If she can't understand that, then maybe you both need a cooling off period before discussing it again.

Just turning around from one conversation, saying 'hi' and a kiss and turning back with no explanation can feel like a dismissal to the person who got the quick 'hi' and kiss especially if she is used to more.

Next time, let your girlfriend know that you aren't dismissing her. You are involved in a conversation and need to handle it before you can give her your attention. It is a lost communication skill: excusing one's self from one conversation to handle an interruption and going back to the previous conversation.

Rome, not a relationship break. More of a 'break-time' in the school day.