In the north, segregation wasn't codified into law as it was in the south. It was practiced on an individual basis. Blacks lived in black neighborhoods and went to their neighborhood schools. Whites did the same. When white families flee to the suburbs to get away from minorities, they aren't doing anything illegal. Immoral, maybe, but not illegal. And schools aren't obligated to bus in kids from other school districts. I went to an all-white high school in a small town in Illinois. The reason it was all-white was that there were no black people living in my town. If we were to "desegregate" my high school, we'd have had to bus in black students from 30 miles and a county away. Did my school practice de jure segregation? No. Was it de facto segregated? Yes.
The other form of segregation in the north, "red-lining," does go on, although much less than before. Government and non-government organizations constantly send out undercover agents masquerading as home buyers to catch real estate agents who practice this. And if you ever talk to a real estate agent, they'll tell you that what they're allowed to say about certain subjects is very circumscribed. Penalties are heavy--fines, loss of licenses, etc.. Today "red lining" has given way to "steering."
Well, to be fair, some school districts in the North drew lines to make sure that there were overwhelmingly white schools and overwhelmingly black schools.
That was the exception, rather than the rule, however.
Actually, it's more complicated than that. For example, in Detroit you not only had white flight but also black middle-class flight.
Though there certainly was a racial element to urban flight, people of any race who had the ability to do so fled places like Detroit after they went to hell in a handbag. There are several perfectly safe and respectable "black" suburbs of Detroit.