Curt Ehrenstrom never saw it coming. But midway through teaching an advanced placement physics class Wednesday at Mount Carmel High School on Chicago’s South Side, the Oak Lawn man learned he had been named one of the best teachers in Illinois.

Ehrenstrom won a Golden Apple award, given each year to the top teachers. And when one considers Ehrenstrom is one of 135,000 teachers in the state, that’s quite an honor.

He won $3,000, which the father of four said already is spent on a bathroom renovation; a tuition-free spring 2014 sabbatical to study at Northwestern University; and a standing ovation from the entire student body gathered in the school’s gymnasium.

Through it all, Ehrenstrom had one of those “this can’t be happening to me” smiles on his face.

“I don’t think I do anything extraordinary. I am a teacher and I love what I do,” he said.

He was grateful, however, that he was nominated by Mount Carmel students even though he told them, “I’m mean to you, day in, day out.”

His family and some friends attended the event.

While the assembly came as a surprise to Ehrenstrom, it was not surprising to Father Tony Mazurkiewicz, Mount Carmel’s president, that Ehrenstrom won the award.

The Golden Apple, he noted, goes to those who “have a passion for teaching.” And that applies to Ehrenstrom, who taught physics and was Mazurkiewicz’s football coach when the latter was a sophomore at Mount Carmel.

“He makes it exciting to learn,” said Mazurkiewicz, a 1992 graduate.