I see that you're new here. Welcome aboard.
As difficult as life can be at 17, you can be thankful that you're not older and some of these forum members. It looks like most of these responses were either unoriginal cliche's, or someone way too focused if this is under the proper forum heading.
All these members were 17 at some point. If most can't even offer a 17 year old male on the internet some authentic thought-out advice... then can you imagine how they operate when with an actual woman and in person? No matter your outcome with this girl, I'd bet that you'd do better than most that have (so far) replied to your posting.
It's been a long time since I was 17, but I can remember certain awkward situations. In an ideal world, we could just walk up to a girl and say... "hey, I think you look nice and I'd like to get to know you." Unfortunately, it doesn't normally work this way.
At my age, I go for more of a direct approach. This will come with practice to you, but it will take some time. Back when I was around your age, I found that using a "warm up" method worked best.
I'm quite a bit older and realize that a younger generation might word things differently than I would, but this will give you an idea.
Keep it very casual. Start of with some small talk and then build up to something ambigious... go with something like.... "Hey, I was thinking of checking out _______ (insert movie title) ....have you seen it?" or... "Hey, I heard that _____ (insert coffee shop name or name of a new restaurant) is pretty cool. Have you been there yet?
As you can see above, you haven't asked her out. The point of this is to gauge her response to your question. If she smiles and/or her eyes get a little wider, it's a good sign. If she suddenly looks down or away, it likely isn't a good sign.
If her body language doesn't appear favorable, you can respond with ..." I've read mixed reviews on the movie so I thought I'd ask someone that I know." Or... "I read that the place seems cool, but sometimes you can t trust a review so it's good to ask around." And then, change the subject to small talk and then casually say something like"okay, well, I'll see you around." The reason that you want to end with small talk is so you don't appear phased by her reaction. You want to appear cool and calm. Or as the newer generation says... "you want to appear chill."
If her body language seems positive, you could follow it up with..."I was thinking about heading there (if a coffee shop) after school you should come with me." or... "I was thinking of finding out (if a movie or restaurant) on ____ (insert day) . We should check it out." The wording method was deliberate. You don't want to say... "would you" or "do you." Instead, you want to keep it confident and lead by saying 'we should."
Naturally, you'll want to use the type wording that fits you best. Back when I was around your age, it seemed like asking a girl out could wreck my week if she didn't say yes. The more practice you get, the easier it becomes.
Good luck to you and feel free to follow up and let us know.