New York City’s public school system could save $2.4 billion and alleviate overcrowding if education officials think outside the box instead of drawing up costly construction plans, according to a new study.

The report by the Citizens Budget Commission said the Dept. of Education spends too much money on building new school facilities instead of better utilizing the space it already has to address overcrowding at some schools and under-utilization at others.

“More flexible and cost effective approaches, including rezoning, re-purposing available seats, and altering admissions policies, are underused by the DOE,” the report said.

Rezoning school districts would provide the flexibility to swap students from crammed schools to under-utilized facilities with empty seats.

The DOE spent $9.1 billion from 2005 to Sept. 2018 on new schools, creating an extra 98,302 seats for students.

But DOE data still show that 618 out of 1,413 schools are pegged as overcrowded.

“Historically, the strategy has been to try and build yourself out of the problem,” CBC’s president Andrew Rein told The Post.

“What gets lost in the tradeoff is that school construction costs a lot of money.”

DOE spokeswoman Danielle Filson said, “We’re committed to reducing class size and alleviating overcrowding and have already implemented many of the recommendations in the report.”