Colorado districts with large numbers of students refusing to take state standardized tests could face federal sanctions, after the U.S. Education Department Friday rejected a request from Colorado education officials to hold school districts harmless for high rates of opt-outs.

Federal officials said in a letter to Colorado Education Commissioner Robert Hammond that not holding districts accountable for students who have opted out of tests will hinder efforts to improve schools and reduce inequities.

Districts are required by law to test all students in grades three to eight each year and all students in high school at least once. Federal officials have said districts could face sanctions, including the potential loss of federal funds, if fewer than 95 percent of students participate.

But in some Colorado districts, far fewer than 95 percent of students have taken standardized tests so far this year. In Boulder, for instance, district officials estimate that 47 percent of high schoolers, 14 percent of middle schoolers, 9 percent of students in K-8 schools, and 6 percent of elementary schoolers did not participate in the first round of spring tests.

As opposition to testing reached a boiling point, the State Board of Education voted in February to exempt districts from penalties for having low student participation in standardized tests.

The Colorado Department of Education subsequently requested a waiver from the federal education department that would effectively allow districts to not count students who had been opted out of tests toward that required 95 percent minimum.

In the letter to Hammond, Assistant Secretary of Education Deborah Delisle wrote that the federal Elementary and Secondary Education Act does not permit certain students or percentages of students to be excluded from testing and does not permit state education agencies to exempt certain districts from accountability requirements.

“High-quality, annual, statewide assessments provide information on all students so that educators can improve educational outcomes, close achievement gaps between subgroups of historically underserved students and their more advantaged peers, increase equity, and improve instruction,” Delisle wrote.

U.S. Education Secretary Arne Duncan said earlier this week that the federal government might have to intervene if states did not address rising numbers of students and parents refusing standardized tests.

When the state board voted to give districts a pass in February, Hammond advised that “districts still need to engage in good faith efforts to test all students in accordance with state and federal law and maintain documentation of parent refusals.”

Hammond announced his retirement earlier today.

The federal education department did agree Friday to grant the state flexibility on certain testing requirements that would reduce the number of students who are “double tested.”

The letter said that the federal department is still considering whether to grant the state flexibility from a requirement that teacher and principal evaluation requirements be tied to measures of student growth as determined by test scores. The state decided last year that districts could determine, for the 2014-15 school year, how much weight to give growth. Many districts have already acted on that flexibility.