From Martin Luther’s "A Sermon On Keeping Children in School" (1530):
At this point I should also tell how many scholars are needed in medicine and other liberal arts, concerning which a great book could be written and about which one could preach for a half year. Where would the preachers and lawyers and physicians come from, if we had not grammar and the other sciences of speech? They must all flow from this spring. But the task would be too long and too great. I would be brief and say that a diligent and pious schoolteacher, or master, or whoever it is that faithfully trains and teaches boys, can never be sufficiently rewarded, or repaid with any money, as even the heathen Aristotle says. Nevertheless, this work is as shamefully despised among us as though it was nothing at all. I myself, if I could leave the preaching office and other things, or had to do so, would not be so glad to have any other work as that of schoolmaster, or teacher of boys, for I know that this is the most useful, the greatest, and the best, next to the work of preaching. Indeed, I scarcely know which of the two is the better; for it is hard to make old dogs obedient and old rascals pious; and that is the work at which the preacher must labor, often in vain. But young trees can be better bent and trained, though some of them break in the process. Let it be one of the greatest virtues on earth faithfully to train other people’s children; very few people, almost none, in fact, do this for their own.
Source: http://www.godrules.net/library/luther/NEW1luther_d12.htm - (If anyone knows the name of the artist who painted the picture, I'd like to know).