Most schools are vocational schools, where they don't offer college credits, but a certificate. If a vocational school graduate wants to become an RN, she would have to start from scratch, taking college level english, psychology and anatomy classes, which, are on a deeper level than the anatomy learned in vocational schools.
My school is trying to create a bridge program so, we had to take RN pre-requisites before applying for candidacy, so that once a student graduates from there, they will have most of the pre-requisites needed to enter into an RN program. I still graduated with a certificate, but I have transferable credits if I choose to enter into an RN program (or actually, I can transfer them to many other programs, such as physical and occupational therapy, or to a science major). It all depends on your school.