Now, about 30 people who came in contact with the student at Attleboro High School are under a two-week quarantine.
The mayor of Attleboro, a city about 39 miles southwest of Boston, said the student was tested Sept. 9 and that the positive result came in Sept. 11. The student then went to school for the first day of classes on Sept. 14.
Mayor Paul Heroux said a public health nurse for the city spoke to the family on Tuesday and confirmed that the parents had known for days that their child had the virus before sending him to school.
"There's no question about whether or not the parent knew," Heroux said.
The high school's principal, Bill Runey, said administrators learned of the student's positive test result Tuesday.
“I knew that we were going to end up having some cases, but I didn’t expect they would be on the first day,” he said.
The high school is following a hybrid learning model under which one group of students attend in-person classes on Monday and Thursday and another group on Tuesday and Friday, Runey said.
“Long story short, rumors started circulating around town, so someone contacted the bureau of health here in Attleboro and did some checking and found out that it was true, that he had tested positive,” Runey said Thursday.
About 30 students are now under a two-week quarantine to monitor for potential symptoms of Covid-19, he said.
The principal added that contact tracing helped the school identify who might be at risk.
"I was pleased that our contact-tracing protocols we put in place helped us pretty quickly ID and narrow down close contacts," Runey said, "Thirty is still a lot, but if we didn't have greater degree of certainty with seating charts and things like that, we would have had to err on the side of caution for a lot more kids."
This article was originally published on NBCNews.com.