The buses arrived in Lower Manhattan from Harlem, the Bronx and Brooklyn on Thursday morning, carrying thousands of charter school supporters who put on matching red T-shirts and came out to draw attention to what organizers called a crisis in the quality of New York City’s public schools. Some of the smallest protesters, squirming in T-shirts that stretched to their ankles, were less than four feet tall.

“The whole school came,” said Angela Sutherland, whose son, a student at Success Academy charter school in Crown Heights, Brooklyn, stood quietly by her side.

The rally at Foley Square, which included speeches by politicians and a performance by the musician Questlove, was part of a coordinated campaign, organized primarily by charter school advocates, to put pressure on Mayor Bill de Blasio as he and legislators in Albany develop their education agendas in the coming months.

Last fall, as Mr. de Blasio drew ahead in the race for mayor, a similar group of protesters marched across the Brooklyn Bridge in support of charter schools. While Mayor Michael R. Bloomberg was an enthusiastic promoter of charter schools, Mr. de Blasio’s support is far more muted and conditional.