This might be a bit long but really would appreciate if you could give me your opinion about this and maybe an advice or two.

I asked a girl from my school out but Im not sure does she mean "no" or does she think that I was just joking. Of course since she didn´t say yes you could assume that she doesn´t want to go out with me but due to the circumstanses Im not sure.

Ill explain it to you. My friend, who is a good friend of her, texted her and asked her out as a joke but she misundersood and took him seriously. She got really confused about it and I teased her a bit about it (she´s my friend too) like "Now you´re breaking his heart" etc. obviously stating that I wanted them to go out.
For some reason she got sad and angry about that and left us by ourselves. Im not sure about this but I think she might like me and left because I said I wanted them to go out. (Later she finally undersood that it was a joke).

Now that you know the details of what happened earlier in the day It´s easier to understand why I suspect she think I was just joking.
Later that day I was talking with her on facebook and just spontaneusly asked her out. Here´s the chat translated from Finnish

Her: Loooseeer! ( I call her slave ´cause I make her do all the little things for me and she calls me loser because I "cant" do them by myself)
Me: its really mean to call people by names what bad have I done to you, slave
Her: Well sorryy you should learn to do things on your own :p
Me: Too late to apologize you already hurt my feelings.. Ur gonna have to repay me for this
Her: I dont want to you deserved it!
Me: Luckily I wasnt asking your opinion, Youre going to have to take me out for this
Her: whaat, dont you start this too

She didnt say "no" straight up but I assumed that she meant it so I just went on complaining about how cruel she is to turn me down so easily.
The next day a thought a popped into my head and considered that maybe she didnt say "yes" because she thought I was just joking.

So what do you think was she joking or being serious? How should I approach her about it in school and what should I say?

Your English is pretty solid for a Finn. It's in fact better than many Americans.

Anyway, "what, don't you start this too" is not rejection. It's actually her wanting to clarify if you're being serious or not. You should have said you weren't kidding. But as a side note, you should be the one taking her out, not vice versa. Keep in mind girls like a guy in command. My suggestion to you is to say something like "I was joking about you taking me out. But I'm serious about taking you out." Fixes your earlier slip up, and shows you being serious and in command.

_________________
Before she can respect you, you need to respect yourself.

Thanks, propably has something to do with the fact that since young age i´ve read English literature.

So yeah, I didn´t have time to reply in the morning that I would try this in the day so thanks for the advice!
I said that it was a joke that she would be taking me out etc. but, to my surprise, she turned the offer down. No harm done tough as she´s just another fish in the sea and every try builds my knowledge of the game.