DETROIT (AP) — Summer classes for hundreds of Detroit students opened Monday, 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. 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 posted photos of classrooms on Twitter and said 500 kids were being served.

“This is hard! When I visited schools this morning I knew we were doing the right thing for children,” Nikolai Vitti said.

Detroit Public Schools Community District is offering in-person or online morning classes in 26 schools, through Aug. 6.

Students and staff are required to wear masks. The district said classrooms would be limited to 10 to 15 students per teacher. Vitti’s photos showed space between desks.

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

A lawyer, Shanta Driver, said she planned to file a lawsuit to stop the in-person instruction.

“I’m not going back until this pandemic is defeated. There is not a safe way to return to school while this virus is spreading uncontained,” said teacher Benjamin Royal.

Vitti said it’s up to parents to choose online learning or classroom instruction.

“COVID is not going away. Many of our children need face to face, direct engagement,” he said.