Seriously though AandA, if you have never before had any reason to think your DD might be using drugs, I think it's highly unlikely she'd start now. She'll be excited, motivated and will have lots to do - so unless she has an existing problem, she'll have no reason to start and lots of reasons not to.
All the items others have listed are much more likely :)
One note of caution... There is a newish drug that I personally think all parents of teenagers should know about, but generally don't. It is called m-cat/miao-miao/mephedrone, and it is becoming widely available in the UK. It is extremely compulsive, so although it isn't particularly expensive, a young person who gets started on it can spend a shocking amount in a short period of time (my own DS spent around £1000 in 5 weeks :( ) -buying it and other things that become irresistibly tempting when they're off their heads on it.
Again, I emphasise that it's unlikely your DD has started taking it, but if she has, I think you will be able to tell. It has a characteristic smell - nasty citrus - a bit like loo cleaner/de-scaler - which users sweat out of their pores. It makes them look ill - clammy, spotty, glazed-bright-eyed and flushed. It gives mood swings - very happy shortly after taking it, then a crash/very low mood when it starts to wear off. It also has other marked behavioural effects that you would soon notice - it's a dis-inhibitor (users do things they wouldn't normally do) and because they spend so much money on it, users often steal, so you would quickly notice money going missing. More info here if you need it... But I hope you don't! :)