Asking her to be my girlfriend?

I'm in high school, and I've been friends with this girl since March. We get along really well, we sort of just instantly clicked even though we don't have any friends or classes in common (she's technically two grades ahead of me, but only six months older). In July, just when I was beginning to give up hope of her ever liking me back, she told me she had feelings for me, and I told her it was mutual, but we agreed to take things slow because I'm not out yet and neither of us has any experience with relationships.

We have been taking things slow. We've been on what I would consider dates throughout the summer (we've been to the movies a couple times, we went to the midway, we've gone for coffee...) but they weren't officially dates, if that makes any sense. I've been kind of frustrated with how slow it's been, so I tried to move things forward by hugging her goodbye after she came to hang out at my place, and I think she got the hint because she held my hand at the movies the other day.

I really like her and I want us to be on the same page about what our relationship is because right now it's just like "we're... uh... dating, I guess".
So I want to ask her to be my girlfriend, officially. I can't really do romantic gestures like taking her out for dinner because a) my parents would probably suspect something and I'm not ready for that, b) I can't drive on my own yet so it would be awkward asking her to drive or asking someone for a ride, and c) that's not really either of our styles anyway. I was thinking of asking in just a simple, a bit funny way, but I also feel like I'm overthinking it and I should maybe just ask her spontaneously when I feel like it's the right moment.

Any advice?