In some ways, I hate to have to have to use my own experiences to relate to your problem. Mainly because your problem is unique and only relates to you.
But what I will say is that she might be a nice girl and she might appear to be playing it up more for the other guys in your class but your opinion of her is what counts the most.
You said you heard she drinks, well if you only heard it then it might not be true. Don't judge her on something you can't be sure of.
The best thing you can do about this is to take your time over it. The problem with crushes is that they pop out of nowhere and usually make you want to rush into a relationship not really knowing why. Just take your time, talk to her, relate to her, do the things FRIENDS would do.
Later down the line if you feel a connection that nobody else appears to be sharing with her then ask her then.
One last thing; if you get rejected then you should never be unhappy about it. You should always say to yourself 'at least I had the strength to ask'. If you never ask, you'll never know