There is an important legal distinction between tax evasion and tax avoidance. The former means practices that reduce the tax one contributes and which are actually illegal; the latter means practices that reduce the tax one contributes, which are technically legal, but morally dubious, even repulsive. There is an important moral distinction between tax avoidance and proper use of provisions within tax law that attempt to make tax fairer. It is important to keep these distinctions clear.
Some question whether this latter distinction is meaningful. Mitt Romney, for instance, insists that he has paid every cent that he is legally obliged to pay, and not a cent more. This is a common refrain from very rich individuals and massive corporations. Their claims amounts to: I have not broken the law of the land. That may well be a true claim, but it is not the point of the accusation that one has engaged in morally repugnant, even if technically legal, tax avoidance.
Such a legally watertight claim has a certain intuitive ring to it. Why would I not claim deductions for which the law has made provision? Presumably, such provisions were made in order to avoid a potential injustice from which I might otherwise suffer and so my use of them could even be argued to be a moral good, allowing me to dispose of my income to bless others in ways the government could not dream of and for which the government has already planned ahead of time. And put this way, I agree, such moves can indeed be a blessing.
But that there exists legitimate use does not ensure that no abuse is possible. Alcohol has a legitimate use as a good blessing of God, yet there is such a thing as getting drunk. And while the state may legitimately take interest in placing limits of certain forms of drunkenness (such as driving a vehicle while having a blood alcohol limit above a given determinate figure), it will not necessarily legislate against getting drunk and then making a fool of oneself or being rude and obnoxious to one's family while intoxicated. So we can affirm legitimate use while noting illegal extremes and yet still desire to speak of legal - yet morally dubious, even repulsive - drunkenness.
And as with drunkenness, it is not always easy to pick the precise point where a cheery dram with companions becomes drunken offensiveness, and the distinction may not even always be purely a matter of quantity. But when inebriated revellers stagger down the street at three in the morning yelling abuse at each other and waking everyone within earshot (to pick a hypothetical example), then it doesn't take a finely tuned moral compass to determine something is awry.
Likewise, when an individual or corporation is hiding sums larger than most people will make in a lifetime from the taxman's view by pretending to have some business connexion to a microstate whose primary export is being a known tax haven, then speaking of such practices in a very different moral tone to the teacher who claims a deduction for the purchase classroom materials is no great leap of moral imagination.
And when it is revealed that it is likely that at least £13,000,000,000,000 is hidden in such havens (or more than the combined GDP of Japan and the USA), then moral outrage from the teacher who faces worsening conditions due to budget constraints is neither illogical nor untoward.
Someone who said because the law is not interested in the difference between a relaxed pint over dinner and passing out in a pool of one's own vomitus therefore there is no relevant moral distinction to be drawn would be gently reminded that the point of political authority is not to legislate every morally relevant occasion. Neither should we have any qualms about being willing to distinguish between legitimate tax deductions and the egregious abuse of legal loopholes to avoid sharing the burden and privilege of serving the common good through contributing one's fair share.