On the first day of school Monday, it was all smiles, music and balloons outside Success Academy in Rockaway Park following the charter network’s defeat of the powerful United Federation of Teachers last week.

Once again, Success Academy prevailed against Michael Mulgrew’s and the UFT’s obscene attempt to use the courts to sabotage the education of mostly minority charter school kids.

The union’s judicial campaign left parents fearing their children would have no schools to attend this fall.

On Friday, a Manhattan judge tossed the UFT’s bid to block Success from starting classes for hundreds of students at PS 225 in Far Rockaway and the K495 high school complex on Monday.

(Yes, on Monday: One way Success boosts kids is by starting the school year weeks earlier than the regular public schools.)

Mulgrew & Co. shamelessly tried to use the new class-size law to scuttle the city Department of Education-approved colocation in the two underused (half-empty, in fact) buildings.

Manhattan Supreme Court Judge Lyle Frank rightly dismissed the union’s frivolous claims. Public-school enrollment has been falling and there’s plenty of space for all students at the two buildings without exceeding the new caps (which don’t kick in for a couple of years, anyway).

“It is unconscionable that a union representing educators would try to block schoolhouse doors and bar hundreds of children from starting school,” said Success Academy CEO Eva Moskowitz in calling out the UFT.

This follows at least 20 prior failed UFT bids to block or delay Success colocations.

Which doesn’t stop the union from keeping up the harassment.

Since the UFT plans to appeal, Success should ask the appellate court to make the union pay its court costs when the UFT loses again.

If they cared about the children, Mulgrew & Co. would be pushing the DOE to copy Success’ best practices, which have 80% of its students (admitted by lottery!) excelling at reading and 75% proficient in math.

But the only party doing harm to students and families here is the teachers union, wielding its power for the most selfish adult ends, always at the kids’ expense.