11 To the noble end of preparing worthy ministers of the Lord, it is necessary, venerable brethren, to watch with an ever-increasing vigor and vigilance not only over the scientific instruction, but also over the disciplinary and educative systems of your seminaries. Do not accept young men other than those who exhibit well-founded desires of consecrating themselves for ever to the ecclesiastical ministry.(13) Keep them removed from contact and still more from living together with youths who are not aspiring to the sacred ministry. Such intercourse may, for certain just and grave reasons, be allowed for a time, and with great caution, until they can be properly provided for according to the spirit of ecclesiastical discipline. Those who during the course of their education shall manifest tendencies little suited to the priestly vocation, must be dismissed, and in admitting clerics to the sacred orders the utmost discretion must be used, according to the grave admonition of St. Paul to Timothy, "Manus cito nemini imposueris."(14) In this matter it is only right that every consideration should be put on one side that is inferior to the most important one of the dignity of the sacred ministry. It is, therefore, of the greatest importance that in order to render the pupils of the sanctuary living images of Jesus Christ (which is the end of ecclesiastical education), that the directors and teachers should unite to the diligent fulfilment of their office the example of a truly priestly life. The exemplary conduct of those in authority is, especially to young men, the most eloquent and persuasive language to inspire in their souls the conviction of their own duties and the love of virtue.
Source: Fin dal Principio (Vatican.va)