How to start a convo on fb or twitter with a girl you've seen at school?

[axe1you]

What do you talk about? How do you keep the convo going? How do you flirt

[Beach Bum]

Flirting via social media is never the way to start talking to someone. It can be really creepy and awkward. If you have no other way, then when she likes something you like or post about something, comment on it and then start a conversation.
Say she comments about a football play, send her an instant message via facebook about said play and start a conversation, asking if she's a fan, what made her start watching, what her favorite moment was, etc.
If she comments about a movie, send a message about said movie and start a conversation, asking what she liked about it, etc.
Pretty much, whatever she posts publicly about, send her a message commenting about.