This can be a problem for academics, who tend to be trained exhaustively in specific areas, but the problem is more one of desire than ability. You don't need an incredibly deep knowledge of an area to teach it effectively. Rather, you have to be prepared to teach well. Because private schools draw from an elite, well-educated population, most students are already excellent readers, writers, and debaters when they enter middle school. We've got motivated families, open and creative kids, and the freedom to design classes, order books, and implement innovative ideas.
Research is a different question.
But this level of continued scholarship is unusual. Many schools offer some research support, but it is more in the interest of faculty members' keeping engaged with their teaching disciplines than making names for themselves as scholars. This comes as a relief to some, though the lower status associated with being an educator rather than an intellectual can be a problem, particularly after one has devoted years to obtaining a higher degree that is unnecessary for the job. Boarding-school pay is often lower, but that is more than offset by the fact that life at a boarding school involves very few expenses. Experience with adolescents and teenagers is an important preparation for a career in secondary-school teaching, according to several teachers.