Report cards for Montgomery County’s 151,000 students were mailed Friday after a three-day delay that followed a mass recalculation of final exam grades for Algebra 1, according to the school system.

Schools officials said late Friday that they added 15 percentage points to all Algebra 1 exam scores after they became aware that already-high rates of failure had risen markedly.

Erick Lang, Montgomery’s associate superintendent for curriculum and instructional programs, said the main cause of the failure spike appears to have been a loss of instructional time in the spring semester, as teachers prepared students for state exams required for graduation.

The preparation for state exams took two to three weeks out of the semester, he said.

Montgomery’s Algebra 1 course was aligned this year with Common Core state standards. But schools officials said the state test — the High School Assessment, or HSA — includes other topics.

“We have a new curriculum we’re implementing, and that curriculum is not completely aligned,” Lang said.

Lang said that officials added the extra points so that students would not be penalized for a problem they did not create.

Students are being offered reteaching and retesting over the summer.

A letter sent to parents says that schools will be contacting those affected. It advises families to call their schools to inquire about grades or discuss summer sessions.