"hey" "how are you" "what have you been upto"
Depending on how well you know him
"im <your name> whats yours"
"where are you at school?"
* Ask open questions that cant be answered with a yes/no responce
* Comment on his answers (if possible). i.e. "so where do you go to school?" "i go to chatswood high" "oh really im in chatswood all the time/ive got some mates there too etc etc".
* try not to say stupid shit, freak out, run away etc.
* Dont read the above questions like a list
* Dont ask a question like which school are you, if in fact you are talking in the school playground.
If it goes well say something like "well nice speaking to you, i have to get to [the] train*/class/school sport etc (or somthing similar), do you want to catch up another time?
* dont just say train if your not actually going to catch one. lol
hope this helps.