In 2012, the children of Chicago got a desperately needed longer school day after a hard-fought battle between the teachers union and Chicago Public Schools.

It was a victory for the kids, the schools and common sense.

We can’t say it strongly enough: Do not undo that.

The Chicago Teachers Union has put forth a contract proposal that would effectively shorten the school day for elementary students by giving teachers a 30-minute prep period at the start of the day without lengthening the day overall.

Students could still be in elementary school buildings for the same amount of time, the union notes. But they’d likely be spending that first half hour of the day with support staff instead of getting 30 minutes of instructional time with their teachers.

It’s a distinction without a difference. Less instructional time with a teacher is just a shorter school day by another name.

Mayor Lori Lightfoot gets that, which is why she shot down the idea on Thursday.

We think the vast majority of Chicagoans agree with her. We certainly do, and we want her to stand firm as contract talks continue.

Back in 2012, CPS had one of the shortest elementary school days in the country. It was a bad joke on our children.

Then Mayor Rahm Emanuel, in one of his best moves, insisted on lengthening the elementary school day by an hour or more, to about 7 hours. The extra hour adds up to at least five additional hours in class per week — a substantially greater opportunity for real learning.

The CTU cried foul. Education, they said, is more about quality time than the quantity of time. But most Chicagoans could see right through that — kids can’t learn if they’re not in class. The union eventually came around and helped make the longer day work.

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To reverse directions now, even for the sake of labor peace, would be to sell out our children.

A longer day is no educational magic bullet. How that extra time is used obviously matters, too. But an elementary school day of six hours or less — the previous standard in CPS under then-Mayor Richard M. Daley — was a travesty when compared to so many other districts that offered 6 1⁄ 2 to seven hours per day of classroom time.

CTU’s new proposal to scale back elementary students’ classroom time by 30 minutes a day amounts to 88 hours over the course of a school year — more than two weeks worth of work time in the adult world.

Now, consider this: CPS has made considerable academic progress since putting the longer school day in place. And while there are many explanations for that success, the longer day undoubtedly has been a factor.

Test scores are up. More high school students are on track to graduate and go to college once they get their diplomas. The number of International Baccalaureate programs is up, including in elementary schools. More high school students are taking Advanced Placement courses and dual credit courses with community colleges.

Are there caveats we should mention? Sure. Test scores are still too low. So are graduation rates for African American young men. Too few students pass AP exams to earn college credit. And the rigor of some dual credit courses is questionable.

That’s just all the more reason to press forward. Elementary students who are building a foundation for high school need as much instructional time with their teacher as possible.

That said, the union raises a valid issue. Teachers’ preparation time is, indeed, important. Teachers use their in-school prep periods to grade papers, plan lessons, talk with parents and do other work that is essential to good teaching. The work of a teacher doesn’t begin when the teacher steps in front of his or her students. That’s why prep time has become a major issue in the ongoing contract talks.

But if giving more prep time to teachers means taking learning time from kids, we stand with the kids.

The CTU has set a strike date of Oct. 17, in conjunction with unions for school support staff and the Chicago Park District.

If they do walk, we predict they will have horribly misread the public’s support, especially on two key issues.

For one, the teachers have been offered raises of 16% percent over five years. That’s a sweet deal most Chicagoans would grab. If the teachers strike anyway, an awful lot of folks won’t understand.

Secondly, if the teachers insist on any change in the contract that results in a shorter school day, they had better brace for the backlash.

Chicago’s public schools cannot — and must not — go backward.

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