Guys always fall into these big long text conversations with girls then when they speak to them live it becomes a big deal.
I would text her:
Hey I'm used to girls giving me their number but not in such a random way. Well done for being brave enough to do it though. I'll give you a call tomorrow after school about 7"
don't say "can I give you a call" don't say "how are you" or anything like that. Don't get into any kind of text conversation. Don't even ask the best time to call. just tell her you will call her on a specific day and time and that's it. Trust me all the other guys she speaks to will be hiding behind text messaging and you will have the confidence to just say "Hey I'll call you tomorrow" on the first message.
If she text back and says "Hey I'm not available tomorrow" she may suggest another day then you can just call her that day (dont pretend you are busy then start organising another day between you) Just say "ok that should be ok I'll give you a call" and leave it there.
If she says "I'm busy tomorrow" and doesn't suggest another day don't ask "when is the best time to call" say "well let me know a couple of date's you're free and I'll see if I can give you a call"
You can throw a little humour in here if you like. A lot of people here may disagree with this but it has worked pretty well for me. They are all set to have this nice secure text conversation where they get all the time in the world to give an appropriate response and leave it enough time that they don't look desperate then BAM you tell them you're calling them!
Rememer to be the first to cut the conversation on the phone on a high note. And tell her when you'll call her next. Again don't ask when to call her back. Just tell her you'll call her on Tuesday when you're free.
NEVER ASK FOR ANYTHING its bad game
Hope this helps buddy
Man I just keep giving!
Send her smth cocky, funny etc.. I would bust her for giving you her phone number like that, but don`t forget to calibrate IOI for IOI 
Just take it easy. Don`t make it come off like you are all happy bcs she gave it to you. And as Leo said, it does often happen to you, but you mustn`t tell her that. You have to show her with you behaviour that you are used of that