Almost 30 teenagers have to quarantine after parents sent their child to a Massachusetts school despite knowing they were positive with Covid-19, according to Attleboro Public Schools and the town's mayor.

A Covid-19 positive student attended class on Monday, but the school wasn't notified of their diagnosis until the next day, Attleboro High School superintendent David Sawyer said in a letter sent out to families Tuesday night.

Twenty-eight students who had close contact with the infected person have been notified and asked to quarantine for 14 days, Sawyer said.

The school did not identify the student and family.

Attleboro Mayor Paul Heroux and the superintendent were baffled by the incident, the mayor told CNN.

The parents told the city health department that while they found out their child tested positive on September 11, they thought that the child could go to school after quarantining for several days, according to Heroux.

"The parents used very poor judgment, it's very frustrating," Heroux said.

"The school department did everything they were supposed to do".

Sawyer told families that the incident was not due to a lack of safety protocol. He said the district's prescreening and contact tracing are in place to help reduce the spread of coronavirus, but ultimately there is no way to completely prevent the spread at school.

In a plea to parents, Heroux asked families to keep their children home if they tested positive or are waiting for test results for coronavirus, and to call the school or health department if they have questions.