Experts advise people to take the bacteria seriously and take steps to prevent its spread because it can be deadly, although it usually is not.
They also note that the number of cases emerging is neither surprising nor alarming.
More infections may be surfacing, in part, because people are on hyper-alert and thus are being tested for it.
"The take-home message is this isn't new," said Kathleen Harriman, chief of the surveillance, investigation, research and evaluation unit of the state Department of Public Health.
"We're seeing more and more infections, so it wouldn't be rare to see a couple cases in a school at the same time," she said.
A teacher at Sequoia Middle School in Pleasant Hill was diagnosed with the bacterial infection methicillin-resistant Staphylococcus aureus, or MRSA, said Sue Berg, Mt. Diablo school district spokeswoman.
The teacher remains at home. Custodians scrubbed down the campus Thursday.
In Pleasanton, the district sent a letter to parents announcing that a Harvest Park Middle School student has been treated for the infection and cleared to return to school.
Cases of MRSA also have been reported in recent days at Freedom High School in Oakley and Northgate High School in Walnut Creek.
In a sign of the heightened awareness, portions of College Park High School in
Antibiotic-resistant strains of staphylococcus aureus, or staph, have been a problem in the nation's hospitals since the 1960s. Patients sometimes pick up the infection while being treated for other conditions.
In the 1990s, a slightly different strain began to spread into the community, appearing first in jails and among athletic teams where people are in close contact with one another.
Now it has become prevalent throughout the community.
"If you have a skin infection, there's a pretty good chance it was caused by this," said Dr. Henry Chambers, chief of infectious diseases at San Francisco General Hospital.
Staph is a common germ that many people carry in their nasal passages, under their fingernails or on their skin, without getting ill.
About 30 percent of people are "colonized" with garden-variety staph. Probably 2 percent or less carry the antibiotic-resistant form known as MRSA, Harriman estimated.
MRSA is spread primarily by direct skin-to-skin human contact or contact with a draining, infected wound. People with a break in their skin are particularly at risk of picking it up.
It may also occasionally be spread through contact with contaminated surfaces or items. It is not airborne.
MRSA picked up its name because it is resistant to more common forms of antibiotics, but several stronger antibiotics do work in treating the infection.
People who are most likely to succumb to it are those with other medical problems, or those who get it into their bloodstream, Chambers said.
The so-called superbug gained widespread publicity earlier this month when the national Centers for Disease Control and Prevention released a study revealing that in 2005 an estimated 94,000 Americans became seriously ill from it and nearly 19,000 died.
That is more people than died from AIDS, but fewer than the number killed annually by complications from seasonal flu.
Health experts stress that there is no reason to panic, but some parents said they think school officials are playing it down too much.
"Parents have a right to know immediately when there is a contagion on campus," said Sheila Hill, whose son is a sixth-grader at Sequoia Middle School.
"I lost my father to a staph infection five years ago," she said. "It can kill people."
Custodians at Sequoia spent Thursday afternoon scrubbing down parts of the school with rags dipped in the disinfectant Lemon Q.
In most instances, schools with a confirmed case of MRSA do not need to close, said Francie Wise, director of communicable disease control for Contra Costa County.
Wise sent a letter to school officials this week advising them on how to handle the situation.
She recommends that parents take a child to the doctor if they have a wound that becomes red, hot to the touch or swollen.
"What you're looking for is a sore that doesn't begin to heal within about 48 hours," she said.
Schools should make sure they have adequate paper towels and soap in restrooms and have standard procedures for cleaning towels and sporting equipment, Wise said.
In 2005, Los Medanos College in Pittsburg closed its athletic department for several days after four members of the men's basketball team contracted MRSA. Wise said health officials worked with the college on procedures for cleaning equipment and making sure students did not share towels or soap bars.
Last year, at Children's Hospital Oakland, half of the cultures that revealed staph infections were the antibiotic-resistant MRSA.
"The incidence in the community has been gradually increasing," said Lilly Guardia-LaBar, infection control director for the hospital. "For the most part, though, they are skin infections."
Children can be vulnerable because their immune systems are not fully developed. Their hygiene practices also often aren't as good as adults', Guardia-LaBar said.
She suggested that students bring their athletic uniforms home regularly for washing, and that anyone with an open cut cover it up with a clean dressing.
"With your toddlers, have them wash their hands whenever they come in from playing in the sand," she said.
TO PROTECT YOURSELF