I've heard this from close friends and family who taught. All were teachers, all loved teaching, all taught underprivileged kids at challenging schools, and all of them quit teaching before turning 30. Each quit due to the lack of support they receieved from uncaring, petty, and inept school administrations. Teachers can't teach when they don't feel physically safe, or are abruptly reassigned to different classes mid-year, or know their performance evaluations will be sabotaged by jealous senior teachers or administrators who are poor performers themselves yet impossible to fire. These experiences took place in the US, both in large costal and small Midwestern urban school districts.