Thirteen Massachusetts communities, including Boston, Worcester and Lawrence, have had COVID-19 clusters in schools and received state testing services, officials said.

The state Executive Office of Education told GBH News that as of Dec. 1 the state has deployed its rapid mobile testing unit to those schools and ones in Braintree, Acton/Boxborough, Milton, Malden, Rockland, Southwick, Haverhill, Winchester, Billerica and Marshfield. The service has been available to schools since late August.

Harvard epidemiologist William Hanage said the number of clusters should not spark alarm among parents. The small numbers are what he would expect, he said, because research shows lower infection rates among children ages 10 and under.

"A lot of people kind of will say O.M.G. there's a virus among us,” he said. “However, it's important to remember that finding the cases is much more important than not finding them."

Gov. Charlie Baker and administration officials have frequently urged a return to in-person learning, despite COVID-19 rates that are climbing. State education officials have said there are “very limited instances of in-school transmission” and when they do occur, they are “small and limited in nature.”

The rapid response unit will go to districts where there is suspicion that the virus has spread in a school and there are two or more cases in a span of 14 days. If a state epidemiologist agrees, the unit is deployed by the state’s COVID-19 Command Response Center to offer free testing to asymptomatic students, teachers and staff.

Schools may request the unit pending approval from the state. State officials did not name the affected schools, which could be regular public, charter, private or parochial. The state said its testing unit has been deployed in Brighton but a spokesman for the Boston Public Schools said he was not aware of any of the district's schools in that neighborhood using the mobile unit.

Boston Mayor Marty Walsh said earlier this week that he hopes the Boston Public Schools can reopen after the holidays. It has postponed its start several times due to the pandemic. Walsh said that he is also anxious about an increase in cases following the Thanksgiving holiday and is taking a wait-and-see approach. Most students in the city are learning remotely with the exception of a small number of special needs students.

Two Boston parent activist groups — Voices for BPS Families and Massachusetts Parents United — held a rally outside Boston City Hall Wednesday and started a petition to “get our kids back in school ASAP.”

“You cannot continue to jerk families around this way,” Keri Rodrigues, president of the National Parents Union, said in an interview with GBH News. “Who can run their house on a week-to-week basis not knowing if their kid’s going to be in school or not?”

In Worcester, the mobile testing unit responded to a cluster that occurred at the Abby Kelly Foster Charter School, a school that says its mission is to assist parents as their child’s primary educator.

State officials said the mobile testing unit has been dispatched to Braintree and Lawrence twice. Local officials in both municipalities did not respond to requests for comment.

Braintree, 30 miles south of Boston, has had persistent problems controlling the virus. Last spring, outbreaks in nursing homes caused infections to spike. In June, Mayor Charles C. Kokoros was diagnosed with COVID. And in August, Archbishop Williams High School canceled its in-person graduation after a student and parent tested positive.

The parent had attended a religious service at the Catholic school and later tested positive. In the meantime, their child attended a senior breakfast at the Granite Links Golf Club and tested positive the next day. That forced about a half dozen employees of the golf club into quarantine for 14 days, according to The Patriot Ledger.

About a month later, Braintree High School also had two confirmed student cases.

Correction: An earlier version of this story incorrectly identified the Worcester school where the state deployed its mobile testing unit. It was the Abby Kelley Foster Charter School, which abuts the Central Massachusetts Collaborative on New Bond Street.