Anything and everything.
Who's your favorite teacher? Why?
What's your favorite class? Why?
Where'd you get those nice shoes? Shop there often?
How come you're so much sexier than the other girls here? Oh sorry was thinking outloud about Brooke Burke, didn't mean for anyone to hear that..
Ever have one of those dreams where you come to school naked? Ever come to school naked? ...why not? What would you do if I came to school naked?
Did you do the homework? Can I copy?
Do you wear contacts? (They often get this confused as a compliment)
How do you think you did on that test? Cool, I copied.
That stuff is just off the top of my head. Topics don't even need to make sense, just random things off the top of your head. Be a sarcastic, be funny, be arrogant, be whatever you want.
And remember, always try and elaborate on subjects. Ask open ended questions. Look for Pimpologist's posts for great conversing techniques, check the bible for more topics.