I had lots of girls like me when I was in high school, and you know what I did, nothing. Why? Because I was a pussy. Lots of guys are so it's nothing new, but I totally regret it because I missed out on lots of fun adventures with women because I wouldn't do anything to take flirting to the next level. You will never get some flashing green light from a girl that she wants you to step up as a man and make a move, but if the subtle hints are there, which you explained they are, then it is up to you to do something about it. She won't, because it generally is assumed to be the mans job.
You know what she thinks if you do nothing? She thinks you don't like her that way, and she will find someone else. She has given you the invitation, are you too much of a pussy to accept it? I was, and I am not happy with my old self for that. You have desires, and so does she, so act upon them. If you don't she will find someone else that will, and then you will think about this advice, and just be mad at yourself for being a pussy, because the hard truth is that is what you will be.
My advice, man up. She gave you the green light. If for any reason she doesn't accept your advances, simply say you got the wrong impression and leave it at that, be friends or not. Awkwardness is a choice. Regarding boyfriend, just ask 'where's your boyfriend?' when you see her alone, then she will divulge she has one or not. If she says she doesn't have one then, say 'oh i thought you did' 'Well in that case do you want to grab some food after school this week?' something like this. Remember, there is no perfect time to ask, and no perfect way to ask, there is only asking or not.

This thread now requires you to report back to us what happens next week. You've gotten the advice, now it is up to you to create what you want in your life. Good or bad, just go for it!