This story reminds us why Unions, however necessary sometimes, can also cause a great deal of trouble.

A Queens teacher who collects a $100,000 salary for doing nothing spends time in a Department of Education "rubber room" working on his law practice and managing 12 real-estate properties worth an estimated $7.8 million, The Post found.

Alan Rosenfeld hasn't set foot in a classroom for nearly a decade since he was accused in 2001 of making lewd comments to junior-high girls and "staring at their butts," yet the department still pays him handsomely for sitting on his own butt seven hours a day. . . .

The DOE can't fire him.

"We have to abide by the union contract," spokeswoman Ann Forte said.