I simply disagree that it’s a sin … that a thought can be sinful. I completely and consistently fail to see why that should be so.

That may have something to do with the context in which you define or understand the word ‘sinful’. From the limited familiarity I have with your world view, I suspect that context is anthropocentric or human-centered. While it might be claimed that humanism works as a normative ethics—but ultimately it doesn’t—it has nothing to do with how ‘sinful’ is defined and understood, the semantic roots of which is found in the Old Norse (verð sannr at) notion of “to be truly the one (who is guilty).” Under the Christian view of normative ethics, moral culpability is predicated on God as the final ground of moral order, supreme Law-giver, and all-seeing Judge. Being found guilty before man is predicated on breaking human laws and is called illegal, whereas being found guilty before God is predicated on breaking his laws and is called sinful. (And, as said many times before, his laws are based on his nature and character; i.e., they are not arbitrary, a mere function of divine fiat or power.)

And so as beings created in the image of God (Imago Dei), that’s why our thoughts can trigger our moral compass or conscience. For example, if I am always scheming and imagining ways by which I can plunder the cash from my best friend’s safe, when he commends me to others as an example of someone who is morally upright, I feel pangs of guilt. My conscience convicts me, even though I never did anything bad physically or outwardly, for my thoughts were sinful.

Are you one of those who thinks that something is morally wrong only if it causes harm to another? If not, then I am glad. But if so, then I’d have a moral dilemma for your contemplation.

“To write about how she would sell her soul to the Devil, what trade she would make, would require her to wilfully sin.”

The Sermon on the Mount and the Decalogue both condemn you for what you think. Not how you act, but what you think. Now, I fully recognize that for a believing Christian, writing a flight-of-fancy, fictional, hypothetical account of how one would go about selling one's soul to the Devil might be troubling (indeed, that's why I suggested a compromise in the assignment). However, I simply disagree that it's a sin, and think of it more as exercising the imagination than anything else.

You seem to think (with biblical authority, obviously) that a thought can be sinful. I completely and consistently fail to see why that should be so.

“Die Gedanken sind frei, wer kann sie erraten,
sie fliegen vorbei wie nächtliche Schatten.
Kein Mensch kann sie wissen, kein Jäger erschießen
mit Pulver und Blei, Die Gedanken sind frei!”

But I agree totally with Ryft. In both cases a power testimony for Christ could have been written. It may still have got a failing grade but the principles of the student would not have been compromised. Like Ryft said it's hard to expect the student to see this opportunity and respond in this way, but it would have been nice to see the parents with a bit of apologetic vision instead of retreating into the panic room whenever presented with anything non-Christian.

“I think she should have accepted the assignment in the first place,” you said. “I don't see what's wrong with it.” First, moral properties do not depend on your intellectual approbation for them to either inhere or be valid; something can be morally egregious even despite your inability to grasp its particular wrongness. But I am confident that you already know this.

Second, it was wrong because the assignment required the girl, on pain of receiving a failing grade, write an essay that would compromise the lordship of Christ in and over her life. To write about how she would sell her soul to the Devil, what trade she would make, would require her to wilfully sin. It does not matter that it amounts to little more than a thought experiment; that is precisely where sin begins (e.g., Jas. 1:14-15, Matt. 5:27-28). Even the Ten Commandments prohibited coveting.

Third, I agree that she should have accepted the assignment, but for reasons radically different from yours. I think she could have written a powerful testimony for Christ in such an essay, couching it in terms of how there is nothing anywhere on earth more valuable to her than Jesus Christ—i.e., there is not anything the Devil could offer her that she would trade for, since she already possesses what has incontestable value. But she is, after all, just a high school student; I can understand how a teenage girl might not notice that opportunity. I wish her parents had.

Should educators accept every student's refusal to do any assignment that might upset them on religious grounds?” you asked. Perhaps not uncritically, but then what if the parents likewise object, such as our case here?

Oh, and the school did finally give her an acceptable assignment.

It is a school's job to educate, to be sure. However, I fail to see how it is the school's prerogative to place students in a position where they are required to compromise on their conscience in order to obtain a passing mark.

A variation of the topic ought to have been offered.

Back on the issue, you would think that variations on the assignment topic would be offered to cater for people's conscience and to offer them the opportunity to still produce a work that can be graded. That to me would seem fair – and certainly in my experience of English Literature essay assignments I have always had various topics from which to choose from.

There's a very easy way in which a majority homosexual society can sustain itself. Namely, artificial insemination. (Or sacrificial heterosexual intercourse, on which, after all, you'd have to grudgingly spend only a fraction of your sexual time.)

Anyway, should the student get a failing grade for refusing this assignment? I think she should have accepted the assignment in the first place; I don't see what's wrong with it. (It sounds innocent enough.) But when she refused to do it on religious grounds, the school could have done the prudent thing, and given her another assignment, perhaps.

But more generally, should educators accept every student's refusal to do any assignment that might upset them on religious grounds? I don't see why that should be. Think: We are all different, but when we go to school we go with a singular purpose: education. There might be many things which offend many students (imagine a black person refusing an assignment to write an essay on slavery in the USA), but from my point of view, that's part of education. Imagine if all tutoring fell right in line with the opinions of the citizenry. No one would learn a thing.