The topic of lying has been discussed on SI before, and you might like to look for the old threads.
I believe that mguldner has made a good point, in line with John 8:44. There is bound to be a way that you can tell as story as a narrator, getting inside the mind of someone who thinks it's okay to lie, and who justifies it to himself because within his (or her) own personal philosophy, lying is okay (the means) if 'the end' justifies it.
There is no reason at all why you shouldn't bring out this conflict with scripture, within your story, quoting 1 Cor 1:21 For after that in the wisdom of God the world by wisdom knew not God...' and Col 2:8.
In other words, instead of creating a character with the perspective he [i][b]believes[/i][/b] lying is okay, you can have your character lying to please another character, against a background of knowing scripture forbids it.
After he has lied, an alternative to him repenting, would be for him to slide away from God, the more he denied truth (Rom 1).
I had a Lutheran neighbour, once, who was doing a part-time degree in English. The class was asked to read a book the philosophy of which she found profoundly offensive, even though it's a classic, so, she asked the acting Head of Dept if he could give her something else, because of the religious issues, and he agreed. The actual Head of Dept was furious when he returned, but she was just extremely grateful not to be battling with hundreds of pages of junk, stuck her nose in the air and proceeded to come top of the class!
My point is, God will honour you if you honour Him. If the remit for your exercise does not specifically state that you cannot quote scripture, then go ahead and take some creative writing liberties.
Even if the remit states you have to write in the first person singular, as if it is you that is lying, there is no reason you could not give an incontrovertibly fictional title, using words such as fantasy, dream, unlikely tale, alter ego and so on, to distance yourself from it. Or, you can simply begin or end it in such a way that it's clear it's not your story, but one told you by someone else.
These days (and for the last at least 50 years), there has been an academic onslaught against truth, which Christians find hard to negotiate. However, it should only make you stronger. Remember, the person reading loves to be lost in the meanderings of another person's mind. Basically, that is what fiction amounts to.
The study of much English literature is only an excuse to escape the bounds of legal decent and honest expressions of reality, into so-called freedom of speech, which allows every kind of pollution the writer permits to escape from his imagination in a manner palpable to others.
My first thought when I saw your leading post, was to suggest you retell Genesis 3 from the serpent's point of view. You can then slip seamlessly forward 4,000 years to the writings of Paul, and quote some of Romans 5, 6, 7 and 8, and 1 Cor 15 'as in Adam all die' for instance - in any order which suits your plan.
Then, your story will definitely take in 'someone' lying because he believes it will 'help' others - according to his own point of view, of course - and by explaining the truth afterwards, the extent of the lie will be highlighted even more boldly. You don't need to offer a running commentary. Just leave it all laid out as your 'story'. It is, after all, an exercise in creative writing, so, you can make the tree of the knowledge of good an evil as beautiful or plain as you like. After all, Eve had never considered that the fruit would be 'good to eat', so, maybe it was quite drab-looking.
You could also make as much as you like of the shock Adam and Eve must have had when their glory left them as a result of beginning to die.
One last thought: 'imaginiation', is about creating images - idols - which are worshipped instead of God. The Hebrew word can be translated 'stubbonness'. In other words 'when they knew God' (Rom 1) implies that no-one who tells a lie, really, deep down on their heart, actually believes it is true. [i]Telling[/i] it (as opposed to [i]thinking [/i]it, is about manipulating another into a false view of reality. If you're cunning, you could get an excellent mark for what you write. Nothing in the remit prohibits you from retelling another's story.
(Sorry for rambling a bit, but I hope this helps you get a handle on how to keep your inner integrity, while complying, and maybe beating the teacher at his own devices.)