Not to a utilitarian, it isn’t.
The discussion has already hit on the primary utilitarian defense – by increasing the poorer students’ immediate happiness by a small amount, you are also lessening their long-term prospects for happiness by lowering the standards of the education they’re going to receive and by enforcing a view that hard work isn’t necessary to get aheaad. Which, if we assume that hard work really is necessary to get ahead (not a safe assumption, but that’s another issue), then over time the students who benefit from this kind of grade inflation are not going to be as successful or happy as they would have had you taught them to actually put their nose to the grindstone when they have to.
Personally, I don’t think it’s a great defense, because it assumes either a disproportionate effect of a very small beneficial effect (the single grade change), OR it assumes that throughout the students’ lives their teachers are all going to be doing this same thing, instead of the arbitrary and unpredictable series of contradictory messages that their teachers really would be sending over a period of years.
The other big objecton, which I think you’re focusing on, is that it simply seems wrong to deny one guy the benefit of his hard work because of the shitty performance of his peers. And the utilitarian offers a similar defense – it’s not that big a deal compared to the happiness the other students will receive. But more importantly, the utilitarian says that, if, in fact, that is the proper act, and it is optimific to punish the innocent student, then it is the morally correct thing to do (and failing to do it is morally wrong).
That should get under your skin, but it’s really not a factor in whether you should be following Kant or Mill. Because the deontologicists say the exact same thing. They just say it about different situations.
This is one serious and, to my mind, unresolved issues about having a moral theory at all – everyone who follows such a theory will make a decision in some cases that our native, innate moral sense will find abhorrent. And they say that’s OK; our innate moral sense is just instinct, and it pales in the light of the superior reason needed to craft the theory. But that assumes that the theory is in fact morally right. And how can we assume this? There are lots of smart dudes with their own moral philosophies that suggest different results to the same dilemmas, so there’s no way on that score to choose one over the other. in fact that primary way that moral philsopohers try to get people to adopt their philosophy is by appealing to the innate moral sense! Certainly that’s the ultimate idea behind utilitarianism – it assumes that the alleviation of suffering is a good, and so seeks it optimifically. Well, I happen to agree with the notion that the alleviation of suffering is good. But there’s no bedrock to that belief beyond my already extant innate moral sense. Of course, there are times when the theory conflicts with innate morality. But when it does, it’s biting the hand that feeds it – like I said already, the only reason to accept a moral philosophy with rules is because you think those rules will deliver a result that you innate think is moral and good. Ergo, a moral philosophy which tells you to ignore the innate moral sense is useless, because it gives you no reason to accept it.
But if the moral theory does parrot the innate moral sense, then what’s the point? Having a moral theory which says “Do what you think is right” is the same thing as having no moral theory at all. And in the efforts to make their theories consonant with the innate moral sense, moral philosophers following Mill and Kant (even Mill himself compared to Bentham) have tried to sublimate the masters’ teachings – the creation of Rule utilitarianism being the most successful example. But it’s the same problem on another level of abstraction – if Rule utilitarianism or any derivative theory still confounds the innate moral sense on occasion, then the above problem remains. And if it doen’t, then why have the theory at all instead of just relying on good ol’ instinct.
Anyway, that has little to do with your question, except (in summary) in this way – you shouldn’t back Kant over utilitarianism because you’ve got a hypo where the utlis would tell you to do something unpleasant. Because 1) there are situations where Kant will tell you the same thing, and 2) if your accepted theory never tells you to do something unpleasant, then you’ve really got no theory at all – you’ve just got excuses that let you act the same way your innate moral sense would tell you to act without any theory to guide it.