Given that our heroine is a member of a class of five students -- admittedly, the girls' class, and a specialized course of study, and I can make make it a small one, but not astoundingly so -- how large is the town they are in?
If this much of the population does the work that this course trains them for, and they are half of one year, the percentage should be feasible to work out. Though I might consider having more than one school in the town. And I still have to work out how many do it. (Not enough to make it easy!)
And then I have to break it down because the town also is divided in several populations, which I have already shown as rather large. Or perhaps they were supplemented by visitors? Still, a non-trivial number must live in the town. . . .