Public school students in Boston will have a direct route to guaranteed jobs with the city’s largest employer, the Mass General Brigham health system, via a new initiative that will pair high schools eager to expand career training with hospitals desperate for workers. A $38 million investment by Bloomberg Philanthropies — the largest gift in the history of the city’s public schools — will transform a small existing high school into an 800-student feeder for the sprawling Mass General system, wh