DETROIT — Summer classes for hundreds of Detroit students have opened, despite a protest by critics who blocked a bus yard and said the coronavirus makes in-person learning too risky.

Protesters held signs and placed tree branches on the ground to try to prevent buses from leaving to pick up children on Monday. WWJ-AM quoted two bus drivers as saying they quit on the spot.

It wasn’t immediately clear how many students, if any, couldn’t get to school because of the protest. Meanwhile, the superintendent for Detroit Public Schools Community District posted on Twitter that 500 kids were being served.

Students and staff are required to wear masks. The district said classrooms would be limited to 10 to 15 students per teacher.

“This can be done,” said Superintendent Nikolai Vitti, who believes summer school is important for students who fell behind when buildings were closed in March.