Students in Prince George’s County will be required to wear masks when they return to school in late August, school officials said Monday.

Monica Goldson, chief executive of Maryland’s second-largest school system, told principals of the decision at a morning meeting.

Goldson said that 50 percent of students who are eligible for vaccinations have received their shots and that she is looking for a higher number — closer to a herd immunity level of at least 70 percent — before considering lifting the mask mandate, according to spokeswoman Raven Hill.

The announcement comes three days after state guidelines were issued in Maryland that strongly encourage — but do not mandate — face coverings for unvaccinated students when they return to in-person classes.

More than 131,000 students attend public schools in Prince George’s County, where the vaccination rate for 12-to-19-year-olds was under 40 percent in June. Children younger than 12 are not yet eligible for the vaccine. School begins Sept. 8 in Prince George’s.

Debate on whether masks are needed has intensified nationally in recent weeks. School systems around the country have said they are preparing for a full, five-day-a-week return to school buildings as the new school year opens.

The school board in neighboring Montgomery County is expected to discuss the issue Tuesday.

At a D.C. Council hearing last week, D.C. Public Schools Chancellor Lewis D. Ferebee said that everyone — vaccinated or unvaccinated — would be required to wear masks in school buildings next fall. The head of the D.C. Public Charter School Board, Michelle Walker-Davis, said the city’s large charter sector would require the same.

Perry Stein contributed to this report.