As a fellow teacher, I must heartily agree with you. I have no problem making eye contact with my students and my principal. I do, however, seem to have trouble with eye contact when it comes to other teachers, strangers, anyone who is trying to connect with me when I don't want to do so, and even, occasionally, my husband. I suspect it has something to do with status, familiarity, and the extent to which a person tries to connect.
I can look my students in the eye because I am a performer, as any teacher is. This may or may not be the real me; only I will really know. In addition, those students (whether consciously or not) already view me based on their previous life experiences. If I am lucky enough to have deep-thinking students, they may actually view me as a worthy person.
I can look my superiors in the eye because they have already judged me. They have deemed me worthy to have been offered this job. I already know that they respect my talents, my abilities, and my personality (even if they find it awkward). Other teachers, however, may be judging me as we speak. I am socially obligated to interact with them, and am equally obligated to compare myself to them (implicitly, of course)
Strangers want either to feel better about themselves for connecting (which means my individual personhood is irrelevant), or truly to connect with others to enrich their lives (which means that I am only a means to an end).
My husband. My husband knows me more intimately than any other person on the planet. And yet he often looks at me as an alien. I shrink from this unintentional judgement, though I know he does love me. Those we are closest to know...too...much. My eyes will bare my soul to anyone who can see that dimension. And when they do, I can't abide it.