ANDOVER — Teachers in Andover who refused to enter school buildings for work last week engaged in an unlawful strike, the state labor relations board ruled Tuesday.

Aug. 31 was supposed to be the first of 10 in-person professional development days for teachers before students start. The Andover School Committee filed a petition with the Department of Labor the following day, calling for a strike investigation.

"These are unprecedented times, and we are not unsympathetic to the Union’s concerns over the health and safety implications of requiring its members to work inside school buildings or its desire to bargain to resolution with the School Committee over these issues before its members return to work inside school buildings," the board wrote. "However, the union cites to no legislation, permission, reasonable accommodation or bargained-for agreement that permitted its members without consequence, to unilaterally dictate where they perform their work."

The board ordered union leaders and members to desist from engaging in any work stoppage.

Three of the largest teachers unions in Massachusetts have told members not to enter school buildings until they are proven to be safe.