While an internship is not required by the Book of Church Order for ordination, it is highly recommended. Candidates and sessions would do well to seriously consider an internship to prepare the candidate for ministry.
An excerpt from an article by Nathan Trice:
"These are what I would propose as the main ingredients of a meaningful internship. But there is one further element needed to make an internship truly successful. At times, it may prove to be the most difficult part of the role of mentoring minister or Session, for it will require serious deliberation and sometimes very sensitive dealing with an intern. I'm referring to the honest, summary assessment of an intern's character and gifts that ought to come at some point during the internship program. Whether an internship is pursued before, during, or after the completion of a man's seminary education, the church and its leaders who provide such oversight and assistance ought to have this question as their fundamental concern: 'Can we see in this man, by virtue of his character and the gifts manifested among us, evidence of God's call upon him to the gospel ministry.' In a day in which the call to the ministry is all too often seen as a merely private matter between God and a man's heart, churches who provide internship programs should take to themselves the difficult responsibility of providing either outward confirmation, or--when necessary--a disapproval of his call to the ministry. In the latter case, the Session, after careful deliberation, and informed by the judgment of the church as a whole, will need to submit its serious reservations that the one serving as an intern is truly called to the ministry. In the former, more pleasant case, the Session will be able to offer a ringing affirmation of his sense of the call. But in either case it is here, in the context of the local church, and specifically a local church that has received a well-rounded representation of the man's character and gifts, that a man's desire and personal sense of call to the ministry can be given its needed external counterpart: the recognition and affirmation of the Church. It is particularly for this reason that I remain convinced that an internship program is indispensable in the preparation for the ministry."
See the complete article: Ingredients of a Meaningful and Successful Intern Program by Nathan Trice in Ordained Servant vol.6, no.2, April 1997.
The OPC has an internship program, which may include some support for the intern and the mentoring session.
OPC Committee on Christian Education