College students are not counted as city residents.There are roughly 150,000 in the city.
The very high BPS per-student cost results from a number of challenges. Bussing kids all over the city. Teachers are paid a substantial premium over most suburbs because of the challenging learning environment. Most school provide free breakfast and lunch. One or two years of free pre-school programs. Seven official languages and the needfor ESL programming. Aging physical plant. Gross mismanagement, with regularly changing Superintendents.
It’s not just white flight, but middle-class flight. Outside of the exam schools and top charter schools, anyone else who can leave Boston often does so.