How do BSN programs work?

Different colleges and universities set up their BSN programs differently.

Some schools have their program set up so that students are are accepted and enter directly into the nursing program as freshman, "pre-req" courses are built into the curriculum, and the nursing courses are spread out over four years.

Other schools admit people as "pre-nursing" majors (or they may call it something different), you take general education courses, including the specific prerequisites for the nursing program, during the first two years, and then you apply for a "slot" in the nursing program, and complete all the nursing courses during your third and fourth years. The schools that do this typically admit more "pre-nursing" students than the nursing program itself can accommodate, so you spend the first two years not knowing whether you're going to be able to get into the nursing program or not. In the last BSN program in which I taught, the university admitted roughly four times as many "pre-nursing" majors each year as the actual nursing program could accommodate, so all those students spent their first two years of college frantically, desperately trying to keep a 4.0 GPA and cultivating the nursing faculty, knowing that they had only, at best, a 1 in 4 chance of getting into the nursing program. I taught one of the pre-req courses for the "pre-nursing" majors, and the tension and desperation in the air in that class was palpable. (It was horrible.) If the day came and you were one of the 75% who didn't get accepted into the nursing program, you were told that your choices were to either change your major to something else or try to transfer into another nursing program somewhere else. (Personally, I thought that was a pretty inhumane way to run a nursing program, and that's one of the reasons I no longer teach there.)

SO, the short answer to your question is that it depends on the individual, specific school.