Depending on exactly HOW this is being explained to school children, in what context, situations, etc, I'd have a tendency to want a private meeting with my child's teacher. Offhand, my reaction is to say, No, it is not OK to lie (white lie or huge lie) "every once in a while." This is a wrong message.
Instead, school age kids should learn some diplomacy. There are ways to respond to someone HONESTLY/sincerely without hurting feelings. Learning how to handle these situations is far more valuable than suggesting the easy or lazy way out.....(by advising that sometimes it's OK to lie.) That simply does not sit well with me and I doubt it does for most parents. Parents, after all, stress honesty from and with their children from the time they are toddlers. Let's agree to stay on track.
This "lesson" should be an opportunity to teach children life, social skills. When a person asks a question like the one you mention, (Do I look fat in this dress?) it should be recognized as being asked for strictly "confirmation" to the answer that's "hoped for."
We might think that in a case like this, what can a tiny lie hurt? Frankly, in Human Behavior 101, we will learn that lying may be developed through practice to become an unacceptable HABIT. Can you see where this makes sense to acknowledge? Children should be offered the chance to mature and use tactfulness rather than being less than truthful. In other words, an HONEST response in this particular case might be, "I think that's a pretty dress that looks nice on you." Both parties should feel just fine with a truthful response. No need to deceive or be insincere.