In order to take your casual flirty friendship to the next level, you will need to ask her out on a date at some point. Ask her out in a way that is casual and low-pressure to increase her chances of saying yes and reduce any awkwardness if she says no. Try something like:  "I have a spare movie ticket to [that movie she wants to go to] on Friday night. Are you busy?" "I don't know if you have any interest in going, but I usually go to the state fair to be slightly less bored than I'd be at home. You want to be my partner for the day?" "So that big football game is coming up. Do you want to help me celebrate school spirit by cheering for the other team?" While you can ask her out in a way that is casual, you may also want to plan a date that will really wow her. Think about what you already know about her to plan something she would enjoy. Or, you can just ask her what she would like to do. She may appreciate your consideration of her desires as you plan your first date. Showing that you have her comfort and enjoyment at the top of your list will make it more likely that she will want to go on more dates with you. Watch for little ways that you can ensure that she is having a great time all through the date. For example, if she seems cold sitting outside at a café, ask if she wants to head inside to warm up. Or, if the waiter forgot to bring her a drink refill, get the waiter’s attention and ask (politely) about it or just go to the bar and get it for her yourself. After you have had a few dates, let her know how much you enjoy spending time with her. She might feel the same way about you, but you will have to speak up at some point if you want to be sure. Try saying something like, “I always have such an amazing time when we go out. You’re really fun to be around.” Or, be more specific and say something like, “You have the most beautiful smile. It always brightens my day.” Getting a girl to fall in love with you takes time and energy. Even if your first date was amazing, you can’t expect her to be in love with you already. Keep taking her out on amazing dates and enjoy the process of getting to know each other. The relationship will have a better chance of thriving if you let things happen rather than trying to make her fall in love with you right away.

Summary:
Ask her out on a date. Plan a special date with her preferences in mind. Be an attentive date. Let her know how she makes you feel. Keep dating her and getting to know her.