I’m a BS parent, not a teacher like @cameo43, so my perspective may be different. At our school, most of the teachers have at least a master’s degree in their subject and some have PhDs. Not so many with just a BA/BS, so your niece’s degree should be helpful. None of the teachers at our school have no teaching experience at all but quite a few have university/college teaching experience. (I know that some schools will hire directly out of college, especially if the candidate can bring another skill, such as coaching, that is needed.) Your niece may be able to build her resume a bit as a TA or adjunct prof., which could be helpful at a number of places. Because many BS communities are small, teachers who have the ability to teach biology or chemistry or environmental science are valuable because it allows the school to respond to interest, so she should highlight that.
In our neck of the woods, very few of the private school teachers have public school experience, and I think that’s because the path to (public) certification is so different. Public school teachers have to take more education courses but need far less expertise in their subject area. With her STEM background, teaching in a public school is probably an option, and there are some shortcuts to getting there (should it interest her). While it has different challenges, I think the boundaries are much clearer, so if the idea of being part of a community for so much of every week seems a challenge (it would have been for me, at least prior to having kids), she could consider that.
But mostly, I think your niece needs to be onboard for the whole, immersive BS experience. At our school, the teachers don’t teach that many classes, and classes tend to be small, so the number of students (and hence papers, tests, labs, etc.) to grade is far more manageable. However, it’s expected, as @cameo43 mentions, that teachers are invested in their students. I’m always amazed at the teachers who show up at games simply to watch a student play! (Or a play, concert, etc.) And of course, they take on many roles, such as coaches, advisors, dorm parents, club sponsors, etc. And they’re available for consultations, tutoring, etc. Our school has quite a few teachers who have been there for 15, 20, 30 years, and ALL of them claim to do it because they love the kids and they love to teach. As a parent, I don’t think that’s just lip service – I’m regularly wowed by how much joy these folks bring to their jobs and how much the kids like them. I also wonder how they keep up their energy!
Without having had the job, but having watched the successful, long-time teachers, I think that their values are really aligned with the mission of the school. I’m not sure that’s an opportunity everyone gets for the first job, but I’d encourage your niece to dig into that a bit as she looks. I’d imagine that it’d be hard to live in a community that didn’t really reflect your values – if you’re a believer in equality but the kids of big donors are treated differently, for example, that could be incredibly irksome. And while the pay is typically not terrific, if it comes with housing, meals (and sometimes tuition for your kids and “insta-family” for them), and a short commute, it’s not all apples to apples. But as pointed out, it’s a labor of love. Truthfully, I envy the teachers at our school – what they do has meaning, and they have some great opportunities to connect with folks (including other faculty) in a way that is truly gratifying. I can point to several who have really made a difference in my son’s life. Not sure anyone would say that about me or any of my corporate co-workers!
One other thing your niece might want to consider… I’ve noticed that a number of the international schools have openings from time to time, so if she has wanderlust and is looking for a way to build her credentials, that just might be a possibility. Not the BS experience, but not public school either…