limits to parental authority?

To earn my daily bread I teach at an International School...and this is a peculiar niche in the cultural fabric of the world.

My daily experiences are often troubling and leave me with a sense of incompleteness, as if I had entered a sort of whirlpool where I must watch other people drown, all the while doing the best to prevent my own head from going under. Ugliest of all, of course is seeing other people (metaphorically) drowning because someone has pushed them under.....

All these generalities boil down to one particular experience involving a student and another teacher. The issue is whether one should take into account the possibility that a parent uses corporal punishment at home. Should that make a teacher hesitate to give a bad report about student "behavior" to a parent? This particular incident is more complicated because I doubt my colleague's assessment. Moreover, I doubt the efficacy of having an official report of less-than-ideal deportment.
My colleague shocked me when she took the attitude: whatever a parent does at home is none of my business.
On the contrary, if you know your actions have certain consequences, you are responsible for those consequences. And, unless you approve of corporal punishment in the home--and you'd better have arguments for that--then you'd better think twice.
My colleague's approach was, in short, irresponsible.

Recommended article: Nicholas Humphrey, "What shall we tell the children?" http://www.humphrey.org.uk/papers1998WhatShallWeTell.pdf