But administrators hoping to win $10,000 (pound;6,000) from the firm considered it no laughing matter and suspended Mr Cameron, provoking a national debate over the commercialisation of public schools.
Now Mike Cameron's suspension has become a coast-to-coast controversy, and the 19-year-old - who says he never intended to make a political statement - has become a hero, invited to appear on network television talk shows.
"I really didn't do it for any kind of malicious reason," said Mr Cameron, of Evans, Georgia. "I thought it would be funny." Greenbrier High School officials didn't see it that way. They had invited the executives as part of a national contest for schools to come up with creative ways to distribute promotional material.
A Coke executive taught marketing, a chemistry class analysed the sugar content of Coke, home economics students used a Coca-Cola cake recipe and the whole school spelled out "Coke" for a photographer for the "Coke-in-Education Day".
It was while the photograph was being taken that Mr Cameron stripped down to his blue-and-white striped Pepsi T-shirt. He was immediately suspended by the principal, Gloria Hamilton. "It's not a Coke-Pepsi war issue," Ms Hamilton said later. "It has nothing to do with that. It was a student deliberately being disruptive and rude."
But Mr Cameron said the principal had told him privately "how important that day was to the school and that I might have cost the school 10 grand".
Under the harsh glare of national criticism, school officials later withdrew Mr Cameron's suspension. "The penalty didn't fit the crime," said superintendent Tom Dohrmann, who also made a reference to the US Constitution's guarantee of free speech. "We support the students' First Amendment right to wear the shirts, as long as it's not disruptive." And many critics said the story showed American schools have become too subservient to corporations. "We do teach independence in this country, don't we?" asked Hazel Lanier, whose granddaughter attends Greenbrier High. "The last thing we need is to teach conformity."
Meanwhile, Pepsi made a $5,000 (pound;3,000) donation of its own to the school, which is about 133 miles from Coke's world headquarters in Atlanta.
"Without knowing all the details, it sounds like Mike's obviously a trendsetter with impeccable taste in clothes. We're going to make sure he's got plenty of Pepsi shirts to wear in the future," Pepsi spokesman Brad Shaw said.