So after all the other lessons, you've got a great conversation going! Congrats! She's laughing smiling, you're listening and being honest... it all looks pretty good!
Once you've made friends and things are looking good, of course don't be afraid to start conversations with her when you see her! But - make sure you know the line between having good conversations when you see each other and talking to her all the time, whether she looks busy/preoccupied or not.
If she looks like she wants to leave or is losing interest, take a step back (not literally, obviously) And think about it - are you over crowding her?
Of course, I am never one to endorse the '3 day rule' (For those who don't know, this is a 'rule' saying if she gives you her number, you wait 3 days to call or text). Don't do that, she'll assume you're ignoring her and with teenage girls, that does not tend to go well.
Overcrowding can put a girl off very quickly. Do you like it when a girl is talking to you all the time and constantly craving your attention? No? Well neither do we! If her body language is closed and she is giving short replies to you, but then goes to her friends and is completely normal, you might be over crowding her and that's not a turn on. Another way to tell she's not willing to talk right now? Watch to see if she's fiddling with her clothes, looking away, or moving away whenever you move closer.
Be friendly and have good talks, but know the line!