I usally comment (positive) on the strangers clothes or looks.
It's an easy way to start an conversation, if you're about to talk with a total stranger.
But an answer to your question, i would talk about school and class. That you KNOW that you have in common, so it's an easy way to begin the conversation. Listen to the answer, ask follow-up questions. There you go.
If you didn't pick up a intrest from the school-conversation, just ask "Apart from lalala, what do you more like to do?"
And do you have any gossip, PLEASE TALK ABOUT THAT. It's a stupid way to make friends, BUT IT WORKS SO WELL.