Saying hi in public
When my cousin was about 10 he was playing outside with friends. My aunt, before pulling in the driveway, rolled down her window and said hi. “Mo-ooom, why’d you embarrass me like that?”
Kids. Adults. Hello. In public. Iffy proposition. I say hi to my students. Quick, polite, direct. I greet their parents the same way, although slightly friendlier, offering an opening to a quick convo, (if appropriate).
But what about “once removed”? 3 times in the last weeks I have encountered youngsters, to whom I have been previously introduced – by students. What’s the protocol? Be cool. Make the slightest eye contact and look away. Not a word. We both know who the other is, but we won’t acknowledge it. Or just say hi? Silent seems the safer way to go. But weird and awkward either way. (note, these are teenagers. from other schools. saying hi.)
JD2718 
Eyebrow lift and/or small inclination of the head.
Subtle-nod with the head with forced-closed-mouth-smile and sometimes single-slow/medium-speed-both-eyes-blink-simultaneous to add confirmation (yes i remember, yes you are right, etc).
(the eye blink is integrated into the nod, perhaps dangerous and optional..)
Important to know the culture.. Use at own risk. 8)
Eyebrow lift seams to add to much of a surprise, (oh you). I prefer forced-closed-smile, (more formal).