I'm not a master but I do go to 'highschool' and know a few things.
For an opener, just go with pretty much anything. Anything TOO cheesy get's laughed upon (badly) in highschool, makes you the laughing stock of her next girl chat.
If it was 6 months ago, then she'd be pretty familiar with you already,
so just ask anything really.
'Hey i wanna ask you about _____'
'I haven't seen you around. Who are you?' (Make it really high-energy and fun like)
I'm still a beginner at this, but

at least I'm poppin my 2 cents.
The following thing would be fun,

it's cheeky, a joke and would break the ice instantly if you cancel the awkward moment STRAIGHT away.
Thing about this, you seem anxious. Don't think of the circumstances, think it through in your head if you have to, the start, the approach then BAM. Don't let fear kick in and finish your imagination with bad thoughts.