Hey mate, as you say yourself, you always find a million good reasons not to ask out a girl, well here is one good reason to do it:
what if it works out?
sounds like you generally like the girl, and personally i would go for it in your position, if you're looking to get a date or two with her before asking her to the dance, this is how I usually go about it:
All I'll do is simply to not give her a choice in the matter, this requires a bit of trust between you and the girl, but it seems like you have that end down.
usually I say something like "i just crushed my homework for today/this week, lets celebrate, we'll watch (insert random fun movie)"
usually i do this line over facebook or text it, and its worked like a charm to set up some casual (but sneaky) dates with some of the girls in my social circle that i fairly know but isn't too close with, i can then escalate or get to know the girl depending on what kind of relationship i want to built with her.
the good things about this method is:
1: it's generally accepted to get together and watch a movie (where i am from)
2: The girl usually doesn't view it as a date, which makes you less likely to be rejected
3: since this is only something i use on girls i have met and talked to on several occasions i can root out which movies she likes, and if im lucky there's movie she would really like to see and thus i can prepare.
4: if executed casually, i can quickly scope out if this is a girl i am interested in seeing sexually, socially or on a more serious basis
hope you can use it, good luck with the girl mate!!
ps. 2magic's words are golden, like he says she showing IOI's so she likes you, but remember keep your cool, dont start touching her every 2 seconds and showering her with affection or compliments for that reason, i still fall into that trap sometimes when i get a little eager, everything in moderation mate