City schools interim CEO Tisha Edwards said the district had been bracing for dips. Some schools in the city, which began rolling out the new curriculum as early as 2011, completely abandoned the old one.

She said that the district expects it will take at least three years for test scores to begin reflecting what's happening in city classrooms.

"We have to embrace the fact that we're in transition, and we believe in the Common Core because it's right for children," she said.

Still, Edwards said some data — such as 38 percent of eighth-graders testing proficient in math — is cause for concern.

"Let me be clear: No one here is comfortable with the data," Edwards said. "Transition or not, we still have a lot of work to do."

But for most school districts, scores remained extremely high. The declines were not large enough to wipe out the 30- and 40-percentage point gains that many districts had made since the tests were given first in 2003.

Reacting to the declines, Gov. Martin O'Malley said in a statement that gains over the years had been "tremendous."

"Our new curriculum asks teachers and students to dig deeper into core skills and concepts. A drop in scores does not represent a drop in student achievement," he said. "We will continue to support our students and educators during the next few years as we make a transition that better prepares them to compete globally."

In an unusual twist, Dance said he was most concerned this year not about schools that have shown major declines, but those whose scores have remained the same or increased. Those schools, he said, are the ones that have not yet begun the transition and will face the greatest hurdles in the next year.

"For me the individual school results tell a story," Dance said. "For schools that have the larger declines, they are definitely on the right track. ... I would say keep doing what you are doing."

Anne Arundel County's departing Superintendent Kevin Maxwell agreed that the declines were expected.

"I believe that it does not in any way indicate that we are not capable of the work, that the kids are not capable of the work," Maxwell said.

Anne Arundel County pass rates in elementary math and reading remained above 90 percent despite the drops, as they did in Howard County. The two counties saw the largest decreases in middle-school math.

Howard Superintendent Renee Foose acknowledged some of the county's results surprised her. But, she added, "I've never really taken MSA tremendously serious because there's not a whole lot of utility in them.

"By the time kids test and you get the results we're looking at opening school in a month. Kids took these tests three or four months ago. It's just one data point. We're not in a position to put great emphasis on this."

The teachers union's Weller said she expects test scores to continue to decline while the adjustments are made.

"I think we will see it for two or four years until we have everything in place and students adjust to the new higher standards they are going to be expected to meet," she said.

Baltimore Sun reporter Joe Burris contributed to this article.