Mayor Vincent C. Gray said today that most, but probably not all D.C. public schools will be open Thursday. The announcement at his weekly press briefing caps nearly 12 hours of zig and zag in communications about damage to buildings in the wake of Tuesday’s earthquake.

The most serious situation appears to be at Columbia Heights Education Campus, the combined middle and high school in Northwest, where a structural truss is damaged. City Administrator Allen Lew said engineers are still evaluating the building, but school officials are already looking at the possibility that an alternative site — or sites-- will have to be found for its 1,200 students. One spot under consideration is the University of the District of Columbia, where Woodrow Wilson High School students were housed last year during the renovation of their Tenley campus.

The city announced around 11 p.m. Tuesday that just three schools would remain closed Wednesday. Cracks and missing bricks to the 19-century belltower at School Without Walls in Foggy Bottom and to the chimney at MacFarland Middle School in Ward 4 led officials to cancel classes there. The MacFarland chimney was close enough to Roosevelt, the adjacent high school, that it was also closed, Lew said.

The late evening announcement left the impression that the system’s other buildings were good to go.

But Lew said at Gray’s press briefing that a group of 40 engineers did not finish a canvass of the 125 buildings until several hours later. Reports officials received included masonry damage at the new HD Woodson in Northeast and the truss at Columbia Heights. While other situations seemed to be cosmetic in nature, officials decided to err on the side of safety.

On a 4 a.m. conference call, officials made the decision to pull the plug.

“I think we just got to the point where the most prudent decision was to close the entire system,” Gray said.

Thirteen schools, including Columbia Heights, have been “red flagged,” according to Lew. But not all are expected to remain closed tomorrow: Bancroft, School Without Walls, Roosevelt, McFarland, Langdon, Burrville, Eaton, Cardozo, Lafayette, Ferebee-Hope, Noyes and Beers.

Gray has scheduled a late-afternoon press briefing at which more news is expected about the prospects for school tomorrow.