It's very strange that your pediatrician told you to avoid someone who traveled to Mexico for the sake of your kid's health. That is...not evidence based...to put it mildly.
This would not be a blip on my rader, honestly. Short of some sort of known exposure or epidemic (SARS, H1N1 flu), travel does not usually present a health risk to casual contacts of people who have traveled recently. I have a degree public health and have an above average familiarity with infectious disease protocols and I've never even heard of limiting contact with others post travel outside of known contacts with ill people.
If you're still concerned, I'd see if there's someone at a travel medicine clinic you can talk to about your concerns. They should be much more well versed in the actual risks than a general pediatrician.
Edited to add: the only precaution I would expect the school to take is to ask the staff member to stay home if they have symptoms of any illness. If they are asymptomatic, I would not expect any precautions.