The Bloomberg administration has ordered public-school principals to stop allowing private teaching assistants — whose wages are paid by parent groups — to work in their schools, the Times reports today.

The directive came after a complaint from the teachers’ union, as the assistants are not union members.

Parents raise money to hire the supplementary staff because they’re worried about class sizes for younger kids, which reach as high as 28, “too many kids for one teacher,” said P.S. 77 parent Patrick J. Sullivan.

Henceforth, according to the article by Winne Hu, parents must fork the money over to the school, which must hire the workers through the union.

But union workers earn almost double the pay of the privately hired helpers, meaning that parents’ groups may simply abandon the practice.

Funny. On their own, loosely organized parent groups seem to be doing what the city is supposed to do as the steward of taxpayer dollars. That is, provide reasonable services to city residents and visitors at the lowest cost required to attract competent people to do their jobs well, as the teaching assistants seem to be doing to the satisfaction of the people footing their payroll.

For most jobs, this does not mean paying minimum wage, just as people in the private sector don’t earn minimum wage after gaining some experience and education.

But it often does mean paying less in total costs, including benefits, than what the city’s unions can command by virtue of their political power.

The same situation is true for token-booth clerks at the state-run MTA.

Many riders — the customers — would rather have a clerk than not. And most people likely don’t care if that clerk is paid $12 hourly instead of $24, as the MTA currently must pay, as long as the worker is doing his or her job efficiently.

Maybe the schools decree will spur the parents involved to recognize that they can’t have what they want to spend their hard-earned money on — apparently, lower-paid staffers to help out with the kids — because the union has so much clout over Bloomberg and the City Council that the parents have to pay for other things, including public pensions and benefits no longer available in the private sector.

More likely, though, some parents will just get fed up and leave the city altogether.