It was a classic case of supply and demand.

Entering his senior year at Pittsburgh Allderdice High School, Seth Weidman felt there was demand for an Advanced Placement economics class.

So he decided to supply one.

At least one night a week for nine months, Seth taught college-level economics to a group of his fellow Allderdice students, traveling from living room to living room with his dry-erase board in tow.

Fueled by Doritos, pretzels and the occasional homemade tiramisu, Seth's students in the "Weidman School of Economics" numbered 18, with nine of them eventually taking at least one of the two AP economics tests offered.

Thus far, the results have been spectacular. The students took 12 total tests, and of the eight scores that have come in this month, six are 5's -- the highest possible on a scale of 1 to 5 -- and two are 4's. Depending on what colleges the students attend, those scores likely will qualify them for course credits or advanced standing.

Nationally, fewer than 15 percent of students who took the tests in 2007 scored a 5 and just more than 25 percent scored a 4.

"It feels very satisfying," said Seth earlier this week, toting a copy of Ayn Rand's novel "Atlas Shrugged" and looking tan from many hours of playing summer Ultimate Frisbee. "It was fun."

Though the College Board, which administers AP tests, doesn't track exactly how many students study for tests on their own without taking the high school course, spokeswoman Jennifer Topiel said that the number is fairly low. "It's not very common at all," she said.

Seth, now 18, spent six to 12 hours per week teaching and preparing for the class. He occasionally had to tell his students to stop text messaging and pay attention and told students that they couldn't come to class unless they turned in their homework.

Most of the class members were upperclassmen, though a few were freshmen. One of those, 15-year-old Eva Petzinger, found out about the class after she met Seth at a school juggling club meeting.

Though she thought that the idea of a fellow student teaching an AP class sounded "pretty odd," she agreed to try it.

"At first, it was crazy hard," she said. "I was sure I was going to fail the AP. Once we started reviewing, we just got to the point where answering free response questions was almost automatic."

Eva ultimately scored a 5 on the macroeconomics test. As a sophomore this coming school year, she plans to teach herself microeconomics and take that AP test.

Seth developed a philosophy that if students weren't paying attention, it was because he wasn't making the class interesting enough. So he concocted ways to illustrate economic concepts from the real world of high school.

Eva remembered the time he explained the concept of a positive externality using an example of complimenting a girl in the hallway. Before the actual AP test, she said, he motivated the class by doing 60 push-ups -- one for every multiple choice question on the exam.

Though Seth had done some one-on-one tutoring, he had never taught a whole group at once. And effective teaching, not surprisingly, involved a learning curve.

"When I began teaching, I wasn't very good," he said. "I was not prepared, extremely nervous and unsure of myself."

But Seth not only found his teaching groove, but also unearthed quality teaching materials. He convinced Allderdice guidance counselor Lynn Warner to order him copies of the latest actual AP economics tests released by the College Board.

And an attempt to get a well-known college textbook, "Principles of Economics," proved unexpectedly successful.

He e-mailed the textbook's author, Gregory Mankiw, who is also a Harvard professor and a former chairman of President Bush's Council of Economic Advisers. He also contacted a company called Aplia, which provides online access to textbooks.

With help from both parties, he got access to the textbook on Dec. 13. He remembers the date, he said, because he also found out that day that he'd been admitted to the University of Chicago.

"It was an amazing day," he said.

Seth developed an interest in economics during his freshman year at Allderdice, when he saw a Web site recommendation for Friedrich Hayek's 1944 book "The Road to Serfdom."

Whatever Seth thought he knew about economics was fundamentally changed after he read the book, known as a landmark libertarian text. "It made me see that economics isn't just about a bunch of guys sitting on CNBC," he said. "It's more about incentives. It gives you a cool perspective to understand the world."

He proceeded to devour seminal works on economics: anything by Milton Friedman, as well as more contemporary authors like Tim Harford and Steven Landsburg.

Along the way, he set his college sights on the University of Chicago and the renowned economics department that had once employed both Dr. Friedman and Dr. Hayek.

Browsing in Barnes & Noble one day, he pulled an AP economics prep book off the shelf and flipped through it. He quickly determined that he could easily learn what he didn't already know, and decided to study for the AP test on his own.

At the end of his junior year, he took the AP microeconomics and macroeconomics tests, receiving the top score of 5 on both.

When a few of his friends also seemed interested in studying for AP economics, he offered to teach them -- and thus the class was born.

As the AP test approached, the class was meeting as often as three times per week -- with Seth also doing his school work, studying for seven AP tests of his own and co-captaining Allderdice's Ultimate Frisbee team.

There's no doubt that it was a lot of work, said Seth, but it wasn't all altruistic: not only was it fun to teach the class, but he also gained a deeper understanding of economics.

"If you can't explain something simply, you can't really understand it," he said. "Teaching this class made me really understand a lot of the basic concepts that I'd previously read about."

A few weeks after the AP tests, a group of students gathered at one of the many living rooms that they'd used as a classroom. Seth was expecting an evening watching a Penguins Stanley Cup game, but the class had a surprise for him.

Not only had they gotten a cake that said "Thank You Seth," they had also made class T-Shirts with "Weidman School of Economics" on the front and an economics joke on the back.

Getting that recognition for his many hours of work "felt great," he said. "That felt even better than them beginning to get the scores and finding out they got 5's."