Do I call my former high school teacher by first or last name?

I'm in university right now. I want to get a referral from a teacher in my high school for a job I want to apply to. I don't know what/how to call him. Do I call him by his first name or his surname? I never really communicated with him in high school. We joked around a bit, but we weren't friendly or buddy-buddies as I'm sure some students in my high school were with their teachers. We're practically colleagues now, am I right? I was thinking of asking for my reference through email (is this a good idea) but I don't know how to start my letter. I'm not even sure if he'll remember me. So do I call him by his first or last name?