Aristotle declares this confidently, citing Solon, Anaxagoras, and other sages, as having said much the same before him (VIII.).
In the concluding chapter, Aristotle gives the transition from Ethics to Politics.

Treatises on virtue may inspire a few liberal minds; but, for the mass of men, laws, institutions, and education are necessary.
The young ought to be trained, not merely by paternal guidance directing in the earliest years their love and hatred, but also by a scheme of public education, prescribed and enforced by authority throughout the city.

Right conduct will thus be rendered easier by habit; but still, throughout life, the mature citizen must continue under the discipline of law, which has force adequate to correction, and, being impersonal, does not excite aversion and hatred.

Hence the need for a system of good public training.

Nowhere is this now established and enforced; hardly anywhere, except in Sparta, is it even attempted.