Sumat igitur ante omnia parentis erga discipulos suos animum, ac succedere se in eorum locum a quibus sibi liberi tradantur existimet. Ipse nec habeat vitia nec ferat. Non austeritas eius tristis, non dissoluta sit comitas, ne inde odium, hinc contemptus oriatur.
And so the teacher should adopt before all things the attitude of a parent towards his pupils, and he should judge that he is acting in the place of those from whom the children were handed over to him. He himself should not have faults, nor allow them. He should be strict but not harsh, courteous but not lax, lest the former breed hatred, the latter contempt.
Plurimus ei de honesto ac bono sermo sit: nam quo saepius monuerit, hoc rarius castigabit; minime iracundus, nec tamen eorum quae emendanda erunt dissimulator, simplex in docendo, patiens laboris, adsiduus potius quam inmodicus.
He should have much to say about honesty and goodness: for one who is often warned is rarely scolded. He should be slow to anger, but neither should he ignore things which require correction. He should be clear in his teaching, patient in his work, consistent, rather than extreme.
Interrogantibus libenter respondeat, non interrogantes percontetur ultro. In laudandis discipulorum dictionibus nec malignus nec effusus, quia res altera taedium laboris, altera securitatem parit.
He should answer questions freely, he should ask questions of those who are silent. When praising the speeches of his pupils, he should be neither grudging nor effusive, for the one will lead to weariness of the work, the other produces over-confidence.
In emendando quae corrigenda erunt non acerbus minimeque contumeliosus; nam id quidem multos a proposito studendi fugat, quod quidam sic obiurgant quasi oderint... quem discipuli, si modo recte sunt instituti, et amant et verentur. Vix autem dici potest quanto libentius imitemur eos quibus favemus.
In marking the things which are to be corrected he should not be bitter or abusive, for those who scold in such a way as to imply hatred, drive many from the objective of their study... If his pupils are rightly instructed, he should be the object of their affection and respect. And it is scarcely possible to say how much more gladly we imitate those whom we like.
Quintilian, Institutio Oratoria II.4-8