Im super optimistic on this one. I basically think that the chosen class will be your core customizable roster, and they're the ones who will take on the major roles in the story-telling process.
Then one of two things: either a) you're able to get all the students of the other classes, but later and with some level of pre-built stats, or b) you don't get any other students (except for the other 2 lords maybe). In either case, I also think you'll be able to get the other two professors, and I expect a bunch of non-student characters (essentially pre-promotes) also. Especially if B is the case, since otherwise the roster is sitting at like 10....
If we're assuming a roster of even like 30-40, it basically guarantees that there's some other folks. If Jeralt doesn't get FE-parent-syndromed it'd be fun to get him too