Officials say that the revenue, while small, can still make the difference between having new textbooks — or a music teacher or a volleyball team — and not having them.

“If the alternative is huge classroom sizes and losing teachers and losing qualified personnel, yes, this seems like something we should consider,” said Valery Lynch, 48, a fourth-grade teacher in The Woodlands, Tex., north of Houston. “But I know that it’s a bag of worms, and people are going to ask ‘What’s next? An ad on the classroom clock?’ “

Some schools have been selling advertising space on their school Web sites and in campus parking lots, in addition to the lunchroom and the school buses. An online ad usually generates about $100 a month for a school, according to Jim O’Connell, the president of Media Advertising in Motion, a company in Scottsdale, Ariz., that sells advertising for school districts.

Critics say exposing impressionable young children to ads that appear to be endorsed by their educators is problematic.

“Mandatory education laws are based on the idea that education is good for society, and is good for kids,” said Josh Golin, associate director of Campaign for a Commercial-Free Childhood, a nonprofit organization. “That argument falls apart when you’re talking about mandatory exposure to advertising.”

The companies that help place the ads say that children are exposed to advertising just about everywhere they look anyway, including — for many decades — in their high school yearbooks and sports stadiums. They say that the primary audience for ads on the outside of school buses is adults, not children, and that much of the space is being purchased by dentists, banks and insurance companies.

“School bus advertising is not for the kids in the bus, but for the cars around the bus that see the advertising when they’re at a stop sign or driving down the highway,” said Bryan Nelson, a Republican state representative from Florida’s 38th District, outside of Orlando. Mr. Nelson is sponsoring legislation that would allow school bus ads and direct much of the revenue toward defraying the buses’ fuel costs. “When you think about how many people are going to see those ads, you get a lot of exposure, so we can charge a premium price,” he said.