Here’s another way the city public-school system wastes millions: by insisting on building new schools even as whole buildings sit empty.

A new study by the watchdogs at the Citizens Budget Commission estimates the Department of Education could cut capital-spending plans by $2.4 billion, saving at least $150 million a year in debt service, if it dealt more rationally with school overcrowding.

The changes aren’t rocket science, but as basic as, to quote the CBC report, “rezoning, re-purposing available seats, and altering admissions policies.”

From 2005 to 2018, the DOE spent $9.1 billion on new schools that yielded an extra 98,302 seats. Yet it still lists 618 of its 1,413 schools as overcrowded.

Its $17 billion capital plan for FY 2020-2024 includes $1.7 billion for 28 projects in 11 school districts that have unused capacity. And, in an era of declining enrollments, it aims to build five new high schools for $966 million — spending that’s unneeded if it just repurposed existing space for far less cash.

Politics play a role here: Parents don’t like rezoning, and connected builders and unions score big from school-construction jobs.

And no politician will pass up the chance to show for a ribbon-cutting at a “new, state-of-the-art” public school.

Plainly, all that counts for more than doing right by the kids, and the taxpayers.