San Juan Mayor Caren Yulin Cruz is getting schooled, and the key lesson is: Don't mess with President Trump.

On Saturday, the liberal, self-serving mayor held a press conference in which she accused Trump of being incompetent and then claimed Puerto Ricans were dying in the streets while the federal government dawdled over relief efforts after Hurricane Maria ravaged the U.S. territory.

Trump, of course, didn't act "presidential" — meaning he wasn't the least bit diplomatic and he certainly didn't take the high road. Instead, he did what people really must do when dealing with a bully or a liar: Call them out, hard. He even alleged in a Twitter post that "the mayor, who was very complimentary only a few days ago, has now been told by the Democrats that you must be nasty to Trump."

Of course, it turned out the liberal Democrat mayor was a big booster of Hillary Clinton, and Democrats did appear to decide last week to try to paint the devastation on Puerto Rico as Trump's Hurricane Katrina, which all but bankrupted President George W. Bush of his political capital after his 2005 re-election.

On Tuesday, Trump traveled to Puerto Rico to see the relief effort firsthand. In a briefing with local and state officials, Cruz sat at the table, scowling. The uninformed mayor — as it also turned out, she hadn't attended ANY meeting at the joint field office held by federal officials charged with relief — didn't look at Trump as he spoke.

And Trump ignored Cruz, while praising the governor, Ricardo Rosselló.

The president left Cruz out of his praise for local officials. On Rossello, he said: "He's not even from my party, and he started right at the beginning appreciating what we did." The governor smiled and nodded.

Then he twisted the knife. "This governor," Trump said, patting Rossello's hand, "did not play politics — he didn't play it at all. He was saying it like it was and he was giving us the highest grades."

That's a schooling!