You could do what most people are suggesting and just ask her out. What has worked best with me was just asking if they wanted to grab lunch (or food after school/work) at a nice-ish restaurant (AKA NOT McDonalds) I went with a nice deli that has really good sandwiches. That way it doesn't seem like your pouring your wallet to her, but it also shows that you are willing to treat her to something nice.
Ask her face to face, it shows your confidence, but you also get a chance to see her reaction in real time. If you are far to nervous to ask her face to face then start the conversation by texting (DO NOT ask her out by text) and then meet her somewhere and ask then.
something like this:
you[text] Hey, do you have any plans after school?
her[text] No, why?
you[text] Me neither, wanna do something?
her[text] sure, what are you thinking?
At this point meet her face to face
you: Hey, I was just wondering if you wanted to go out for some food after school?
her: OH YES! where where you thinking?
you: i know a great [insert your chosen location]
her: okay, sounds good!
you: alright! see you later, text me!
her: okay, bye!
you: HOLLAAAAAA A CHA BOIIII! (i wouldn't recommend saying this, but in your head i am sure you will)
...And that my friend, is a grade A guide to making that lady friend a girlfriend!
Good luck. But by now, you dont need luck, you need to find your confidence.