Is this a national rule for diocesan seminaries? I know of at least one religious order seminary in America where this is not true. In this seminary, seminarians can begin immediately after graduating high school, and all seminarians study for 8 years (including the novitiate year), regardless of the degrees that the seminarian had before entering.
In the United States, we tend to get hung up on a certain scholastic standard of discussions. As such, we talk about colleges and seminaries and degrees of all sorts as if priestly training is just some sort of professional accreditation program.
It is much more than that.
A “seminary” is any institution which fosters the discernment and education of men who are considering a priestly vocation and preparing by way of studies and spiritual/human growth towards that possible way of life.
Typically, a “major” seminary is the primary institution of a diocese or religious community which facilitates such an endeavor for adult males. It may take them on for full preparations (the traditional route) which is commonly broken into two major parts, philosophy and theology. In such a seminary, the overall program would generally be 7 to 8 years long. Or it may focus more specifically upon the theological aspects (which is now more typical of most - but not all - American seminaries.) In the latter case, the period is anywhere from 4 to 6 years. Sometimes the major seminary will teach certain students who have already obtained a bachelors degree in additional philosophical studies which they must have as a prerequisite for entering into the theologate, even though the same institution will not take on the average college aged man who must study essentially the same stuff. Nowadays, major seminaries also are often teaching lay students (both male and female) in classes oriented for their entering into various forms of associate ministry without being directed at preparations for the priesthood.
Any seminary institution which exists at a level below the theologate is considered a “minor” seminary. Traditionally, (when major seminaries commonly hosted men in both philosophical and theological studies) this term more generally applied to the many high school level institutions which were operated as a sort of early center for discernment and formation of boys who showed potential and/or interest in religious life and priesthood. Today, with such schools few and far between, the term more frequently applies to college level seminary schools. These schools are houses where men attending university and studying philosophy can discern and receive age/experience appropriate formation. Some of them are formally accredited colleges of their own. Others have associations with a Catholic university. Still others are more loose associations where the men may be living together in discernment, but attend individual schools of their choice. These “college seminaries” (as they are usually referred to) became more popular in the 60s and 70s, as the major seminaries began to break up theological and philosophical discerners in accord with a model which more closely resembled the usual models of university accreditations.
There are, also, forms of minor seminary which are not even at a college level, but neither high school. Such preparatory institutions serve a purpose of better readying potential candidates for the rigors of seminary life and studies in a particular environment. Examples of this would be Chicago’s Casa Jesus, for men from Spanish speaking cultures and Bishop Abromowitiz House for Polish students.
To return to the original question of what colleges will work to become a priest, one should enter a school where he will be able to obtain the necessary philosophy credits (and any other prerequisites) for eventually entering a theologate, if possible. It should also be a place where he can grow as a Catholic and a human being in a way which will direct him towards being a good person and, possibly, a priest. It would be wise to talk to advisors who could help him find a path which is most appropriate. It may be wise to enter a college seminary or explore life as a religious, even at an early age, if one is particularly drawn to that certain way of life and believes that discerning immediately and in such a direct manner will be best helpful.
In the end, however, perhaps the key is solid advice which the late Cardinal Meyer of Chicago once offered to his high school seminary faculty and staff. Asked what was expected of them, he replied thus: “Just give me good men and I will have good priests!”