What are your reactions to eye contact?

I was not exactly sure how to word this question, and actually am not sure if this makes sense, but here goes my question..

I am not talking about making eye contact during a conversation, more so, making eye contact to a fellow co-worker, or classmate, across the room. In a room which includes other people as well. Unlike http://www.fluther.com/67780/what-do-you-do-when-you-make-eye-contact-with-a/, this is not a stranger, but someone you are acquainted with, have made conversation to before, or is aware of their presence and see them almost daily.

What are your thoughts on eye contact to accquaintances? Do you acknowledge them? Or do you just make eye contact, and then just continue on?

Let’s say you are sitting in a classroom, being lectured by your professor. As you gaze about the room, whether listening to the professor or not, you make eye contact with another fellow classmate, will you smile, or is it something that happens too often for you to acknowledge, thus you just continue to look around the room?

Does your relationship with the specific person you made eye contact with change your reaction? If it is someone you care less about, or like less, are you more likely to just continue on? Or if it is someone you are interested in, and you make eye contact with them… will you… make a goofy face? smile? wink?

Is there such etiquette for making eye contact? Please educate me.