So today at school I talked to this girl in front of her friends, then when I left I told her that she was beautiful, and I didn't intend to do this but it just came out of my mouth. So what do I tomorrow tell her.
I would think she would take it more as a compliment. If you just said it casually, like if she asked how she looks and you said that she looks beautiful, it's more of a compliment. If you said it while sitting in a candlelit picnic with a bouquet of red roses and wine, well, that's a bit more of a problem lol
Anyways, don't sweat it, she probably isn't thinking about it the way you are. We tend to be overly self conscious about how we look, what we say and do. So most likely she isn't even thinking about it the way you are...I guess, if you can, act like it never happened?
Hopefully she took it as a compliment...if she didn't it was probably uncomfortable for her...and you. She might act differently around you now. What are you to do? Act just as you did before you said it. Don't avoid her..etc...