University President LAWRENCE H. SUMMERS gives a first-grader a math lesson as principal for the day of Allston’s Jackson-Mann Elementary School.

Surrounded by counting charts, days-of-the-week posters, and illustrated alphabet wallpaper, University President Lawrence H. Summers sits cross-legged on a carpeted floor.

Excitedly, the former Secretary of the Treasury introduces himself to his captive audience: “Hi, everybody. My name is Larry,” he says methodically, stretching the “r” in his name. “And do you know what I do? I’m the President of Harvard. That means that I have a lot of students.”

As the University plans its expansion into its recently acquired 200 acres in Allston, Summers will have to haggle with countless community leaders on the other side of the River. But for now, he’s starting small.

“Now what are we learning today? Numbers,” he says emphatically to the group of six-year olds squirming around him. “And math.”

The classroom is familiar territory for Summers, who received tenure at Harvard in 1983 at the then-unprecedented age of 28. But Summers left Harvard’s halls behind yesterday—spending five hours teaching classes and meeting with teachers at the Jackson-Mann Elementary School in Allston—as part of the Boston Plan for Excellence’s Principal For A Day program. About 600 Kindergarten through 5th-grade students from 26 different countries attend the school.

And some of them greeted Summers with a rock star’s welcome.

“It’s him! It’s him!” one student screams out from a classroom as Summers passes by, oblivious to his newfound fan.

Back in Mrs. Keenan’s first-grade class, the world-renowned economist begins with the basics.

“If I have two fingers on this hand, and two on this hand, then how many do I have total?” he asks.

A rousing chorus responds, “Four!”

“That was too easy,” says Summers, an outspoken critic of grade inflation.

Shaking his finger, he takes another stab: “Okay, one more,” Summers says to the group “Which is more, six and four, or four and six?”

“None of them!” a girl yells out.

The rest of the class is silent, with puzzled looks on their faces.

Summers may have just taught them the basic principles of counting (they’ll have to wait a few years for econometrics), but the class has become restless.