Because the sooner they get out, the more quickly they can become unemployed and start having children:

The policy would prevent eighth-graders from moving to high school if they score poorly on standardized English and math tests or fail to pass certain core classes. The teachers union, principals union, and parent groups have opposed it. Proposed by the mayor in his State of the City address, to be official the policy requires the approval of the Panel for Education Policy at its meeting tonight. Since Albany transferred control of the city schools to the mayor in 2002, the panel has never vetoed a mayoral policy.

Panel members — including the five appointed by the borough presidents and eight appointed by Mr. Bloomberg — have usually lined up behind proposals, ever since four years ago, when Mr. Bloomberg averted a veto by firing two appointees who were set to oppose a policy the night before the vote. That policy, cracking down on so-called social promotion by making it more difficult to move out of third grade, is a model for this one.

Manhattan’s representative, Patrick Sullivan, is set to vote against the eighth-grade policy today. In a statement, the president of Manhattan, Scott Stringer, said he asked Mr. Sullivan to vote no because retaining students “rarely works.”

I understand why parents would want to avoid challenging their children to become educated — and I even get why a borough president would prefer a dysfunctional school system — but what’s the deal with the unions? More kids staying in school more years means more jobs — win-win.