A transit bus picking up primary pupils in Sydney, Australia, recently was adorned with an ad that would make most parents cringe.

The Sydney Morning Herald reported that the bus, dispatched by State Transit to shuttle students to Alexandria Park Public School, had an advertisement on its side for an adult event called Sexpo, with the tagline "The most fun you can have with your clothes on."

The bawdily emblazoned bus was apparently sent on the school route by mistake.

"For school bus services, State Transit takes steps to allocate buses which do not feature adult advertising," transit officials said in a statement to The Sydney Morning Herald. "The use of this bus on this occasion was an error."

Although this unsettling mishap took place in Australia, it raises the question of whether something similar could happen here in the U.S.

Advertising on yellow school buses has been allowed in some states in the U.S., but regulations have been put in place to govern the size, placement and content of the ads.

For example, Tennessee's law on school bus advertising prohibits certain types of ads, such as those promoting alcohol or tobacco, and local district policies have to define what will be considered appropriate advertising.

But critics of school bus advertising have pointed to the possibility that school officials' decisions in restricting ad content could face legal challenges.

A 2011 position paper by the Florida Association for Pupil Transportation (FAPT) cites a 1993 ruling in Boston that the Massachusetts Bay Transportation Authority’s “G-rated” advertising policy violated the U.S. Constitution.

While supporters of school bus advertising say that committees should be formed to set criteria for appropriate ads, FAPT’s paper argues that “such criteria may be challenged in court, and it is impossible for anyone to predict the likelihood of success of any legal challenge to restricting the type of advertising on school buses. … The cost for a state or local school district to defend its advertising policy in court could conceivably exceed the revenue obtained from the advertising itself.”

As with the case in Australia, public transit buses are used to transport students in some cities in the U.S.

Transit buses are often splashed with eye-catching — and sometimes racy — advertising wraps. This shows the need for caution on the part of school districts and transit agencies in cities where students are using public transportation. Are there measures in place to prevent inappropriately adorned transit buses from running routes that transport students?

Here in the Los Angeles area, I recently saw a transit bus with a large ad for the provocatively titled Showtime TV show "Masters of Sex."

As a parent, I know I can't always shield my children from the vulgar material that floats around in our public spaces these days, but it's unfortunate that transit buses are sometimes among the offenders.