OP : Very interesting. I think I might have an answer. To be fair, I probably wouldn't have an answer to this a few months ago. It's smthg I thought long & hard only a few months ago.
People generally believe that one of the 10 commandments is : "you shall not lie", when in fact, no such commandment exists. One should always be careful not to speculate or add to/subtract from the word of God.
Here is what the commandment really says : Exodus 20:16 Do not give false evidence against your neighbor. CJB
Exodus 20:16 Thou shalt not bear false witness against thy neighbour. KJB
Needless to say, there is a huge difference between lying & bearing false witness. So, imho, clearly, The Lord doesn't want us to lie in serious matters. Never. BUT, in extreme cases, such as the one you described to the christian girl in your class, one could potentially lie without necessarily having broken the commandment. Don't know if you're well versed with the scriptures but Abraham lied twice about his wife, saying she was her sister, so that pagans wouldn't kill him & take her for themselves. & Abraham is pretty well-known for walking in God's statutes pretty much all the time.
Now, in reality, Sarah was also his sister... so this further complicates the matter. But still, some people today will say : "a lie is a lie..."
So, it's kinda complicated. There's obviously a thin line between a lie, a half-lie or a white lie.
My take on this is that if one is a true born again believer, he should not worry if he's doing the right thing or not. God will provide & He will understand. If one lies & thinks it's God's will then it's good enough. If one does not lie & thinks that is God's will then it's also good enough. The fact is, that the commandment doesn't have the word lie in it... Period.