Some of the increase might be due to families doubling up in units, but probably less than you think.
I suspect that if someone looked in detail at the enrollments, you'll simply find that more one and two-family homes are occupied by families with children, and less by empty-nesters. The older folks are dying off or retiring out of Hackensack, and houses are selling. And more families with children are living in apartments because of the housing crisis --- mortgage rules are tougher and people can't qualify for mortgages. Plus rents are way up and salaries aren't, so people can't even afford to save money to buy a house. They are trapped, as I am, renting.
Hackensack doesn't need to build a single unit to hit 50,000 population, because the number of people per unit that had been going down for decades is going to go up. Or it has already gone up. Watch when the next census comes out, Hackensack will be at 50,000 or very close to it.