First of all, it is worth pointing out that Plato's moral education is not our mass education today, but elite education. The purpose of moral education is to train politicians and philosophers, that is, the king of philosophers. This is the viewpoint put forward by Plato in his masterpiece The Republic.

Use syllogism to illustrate:

Virtue is knowledge (major premise)

Knowledge can be taught (minor premise)

Therefore, virtue can be taught (conclusion)

There is no problem with the minor premise. The key is to clarify the major premise: Is virtue a kind of knowledge? Socrates first pointed out that virtue is both a good thing and a beneficial thing. Then, if we can find something useful but not knowledge, then virtue is not necessarily knowledge. On the other hand, if we can't find such things, virtue can only be knowledge. Useful things, such as temperance, courage, justice, wealth and good memory, are beneficial because we use them correctly and moderately, that is, according to wisdom. If we use them improperly, they will become harmful. So something useful must be some kind of wisdom. Therefore, virtue is knowledge and wisdom.