PHILADELPHIA — People leave their jobs all the time. Just look at the last few years. The #Great Resignation, and all that. Among the areas hardest hit were schools, and this is a story about an especially bittersweet departure. In 2018, I met Maureen Boland, a ninth grade English teacher at Philadelphia’s Parkway Center City Middle College — a public high school in Callowhill where students take college-level courses along with their high school classes. Boland sent me an email about my work on gun violence, and asked if I’d come to the school to listen to her students read essays about its …