It may just be one of your own little quirks but it is also a very common symptom of an Autistic Spectrum disorder. My 21 yr old son was just diagnosed last week with Asperger's Syndrome, a really high functioning form of Autism (very high IQs). If stressed in any way or if it is a serious discussion of any kind he can not look people in the eye, well maybe for a second if made to. How are you in other social areas? Like:
do you seem to lack empathy for others? Like if someone is hurting do you pick up on it right away?
Are you a pretty blunt, speak your mind kind of person? Do people misunderstand you and get upset for no real reason you can explain?
Trouble managing time? Is it difficult to plan ahead, see consequences, or cause and effect?
Have a hard time following directions? Do you miss sarcasm or have trouble reading between the lines or reading people's facial expressions?
Are you easily taken advantage of? Like do people walk all over you?
ARe you bound by a routine, time or enjoy repetitive activities?
Do you seem hypersensitive to sound, smell, touch, taste, or vision? My son hates to be touched and tenses up. He also notices smells and the new bright lights in our kitchen bug him.
Do you cope with change okay or does it really stress you out?
Do you have hobbies that consume alot of your time? Or interests that you obscess over?
Do you make and keep friends easily?
All these are typical of Asperger's. What you may be dealing with may be just a quirk of yours, we all have them. If you struggle with most of the above you may not be able to help the eye contact thing going on. Don't stress over it, try some of the ideas you have been given and make an effort if you think of it at the time, but don't fret over it. Maybe be even joke about it with the kids but let them know that you are listening, just not looking. I'm sure you are a great teacher and the kiddos love you.... just the way you are (I loved Mr. Rogers!)