He was upset enough to take his case to an employment tribunal where it was argued that, because the post might touch upon matters of morals, it was necessary to appoint a Catholic. The tribunal did not accept the argument, ruled that there had been religious discrimination and awarded compensation of pound;2,000.
Not that Mr McNab could consider himself vindicated since he was left with a large bill for costs and, presumably, the job went to someone else.
The problem met by Mr McNab and many others is that, under the terms of the 1918 Education Act, when the Scottish schools previously run by the Catholic Church were brought within the state system, the Church retains the right to approve teachers appointed to Catholic schools. In practice, teachers who are not Catholics are approved for Catholic schools, usually because a suitable Catholic is not available.
The Church will always require that a head or depute should be a Catholic and many would see it as reasonable that a Catholic school should be led by a Catholic. Others take a more cynical view and say that the Church is happy to accept the efforts of teachers like Mr McNab when there is a shortage of Catholics, but it will toss them aside for promotion when the Church can provide sufficient candidates of its own.
Not surprisingly, the Church takes the moral high ground and argues that it would not be possible to maintain a "Catholic ethos" in its schools without its legal right to approve teachers. So, with the law on its side, approval is a matter of black or white. Teacher X is approved; teacher Y is not.
However, in practice the situation is murkier and causes offence and distress to many.
When teachers are rejected for a post in a Catholic school, they are insulted and confused. They see the rejection as a negative judgment on their self-worth and are angry that priests, often not known to them, can exercise such power. Their feelings are understandable, especially after a period of commitment to the school. If it's any consolation, some Catholic teachers, too, experience the same rejection because of "irregularities" in their marriages.
One wonders how the Church thinks it can continue staffing Catholic schools at this rate of rejection or if it cares about its pastoral responsibilities to the rejected.
Headteachers can be on the distress end of the approval system also. It may ensure, as far as possible, Catholic teachers in Catholic schools but, when lack of approval means the dismissal of the best candidate, the school and its pupils are deprived. Headteachers like to think they are choosing the best staff for a school, but an approval system that only considers a teacher's religious affiliation can mean that a poorer candidate is appointed. How does it help a headteacher to raise a school's attainment if the Church's rigid interpretation of approval is handicapping recruitment?
Because of its nature, the Catholic Church deals with certainty. It is not given to doubts and finds it difficult to listen to other points of view.
In the early 21st century, it needs to rethink "approval" which, as presently practised, can hinder schools and alienate good teachers.
A wider concept of "approval" would include an appreciation of the qualities of teachers who are not Catholics but are committed to the school, and an acknowledgement of the importance of attracting the best staff. Until this attitude develops, Mr McNab will not be the last in the long line of teachers aggrieved and insulted by the Church's stance.
Brian Toner was headteacher of St John's primary in Perth.