ok, I just realized something.. You cant really ask a girl out without know her the least bit, unless its for sex (but thats a whole other topic)..
During recess and lunch, try sit at the same table as her.. And casually add something to their conversation. If she has any interest in you, she'll want to know more about what you have to say.. And from then on, its all good!
There, it didnt cost anything.