Why Is Lying Seen As Bad

After reading Billy Collins's poem, "The History Teacher," some conflicting thoughts come up in my mind.  There is a major debate about the ethics of lying.  About whether some lying is OK if it is meant to protect someone, and at what degree of lying does it become unethical.  In the poem, the history teacher is "Trying to protect his students' innocence" by lying to them.  He does this by taking all of the cruelties out of major historical events, but in doing so, he was not teaching his students accurate information.  The teacher may have good intentions, but does his lying do more harm than good.  In this poem the grade of these students is never mentioned and I feel that their grade could be one of the determining factors in whether the history teacher.  The reason I believe this is because if the children are of a low age and grade, like kindergarten or first grade, the teachings don't matter as much as they can be taught the correct history as a later age.  This way innocence of the children can be preserved while later being taught the correct information when it actually matters to them.  If this were the case then what the history teacher did doesn't seem so bad.  On the other hand, if these students were of a higher grade level, where every bit of information that one learns is vital, the misinformation provided by the history teacher seems much worse.  There is also the thought that there is no point in lying to these kids as they seem to already have the cruelty preprogrammed into them.  This is shown when Collins says, "The children would leave his classroom for the playground to torment the weak."  There is no point in trying to protect someone when they have the problem within them.