BOSTON — Boston public school teachers announced a legal challenge Thursday to the city’s decision to continue some in-person classes, which they allege is in violation of an agreement that requires all-remote learning if the city’s coronavirus positivity rate rises past 4%.

Mayor Marty Walsh announced Wednesday that the next phase of the schools’ reopening plan would be delayed by a week because the city positivity rate had climbed to 4.1%.

But students who had already returned to the classroom, including those with special needs, would continue with in-person instruction.

The Boston Teachers Union said Thursday that based on a memorandum of understanding, in-person work is now optional for all teachers.

The union also said it objected to a suggestion by Superintendent Brenda Cassellius that teachers could be disciplined for opting not to teach in person.

The mayor’s office said it interpreted the memorandum of understanding differently.