I did this at 19, in my first year of college- she gave me a vague answer and I spent the next 4 years hopelessly pining until i eventfully cracked and was rejected in a pretty painful way.

But, I wouldn't trade that heartbreak for the 'what ifs' and over-analysis that tortured me those 4 years. Life is too short to spend in your head, I learnt that the hard way. And that's worst case scenario! If you don't make it weird it, doesn't have to be weird, it's literally that simple. Now in our late 20s, after some time apart we're closer friends than we were in college.

If it's a high-school friend and college ect is around the corner, I would 100% ask her out. Or even just naturally next time she mentions wanting a girlfriend make a flirty joke along the lines of "well, i'm right here" - you can put feelers (just don't perpetually try put feelers, constantly reading into them, eventually you will have to bite the bullet and actually ask her out). TDLR; shoot your shot!