OP, start college, attend for awhile, and then get a job/part-time job/volunteer at the school for a bit. Run into him. When you are leaving for your next job, ask if he'd go out to see a movie with you or something.

By being out of HS and into school (or a job, in general) it separates you from being his old student. In going to the school again, you open up a justifiable reason for your interest in him.

It won't be as easy as if he were 25, but it's still a shot you can take.