The College Board said the lower scores were partly due to the rise in students taking the exam during the school day. These students are more likely to be minority, attend high-poverty public schools and have parents without college degrees. The groups are typically underrepresented on college campuses and might never have taken the test before, said the College Board.

“Those fluctuations we expect to see because our population is changing so much,” said Cyndie Schmeiser, senior adviser to the College Board chief executive.

Since the SAT is now measuring the college readiness of students who previously wouldn’t have taken the test, it is understandable that overall performance has fallen slightly, she said.

College Board officials said the increase in students taking the exam is a good indication that more are considering college as part of their future. The percentage taking it during the school day grew to 43% from 36%.

Overall, the combined mean SAT score is down to 1059, from 1068, out of a possible 1600 point scale for the two sections on the exam—math and reading, writing and language. The percentage of students meeting benchmarks to indicate readiness for introductory college-level coursework slipped to 45% from 47%.

Those not meeting any of the benchmarks increased to 30% from 27%.

Students struggled most in math, with only 48% meeting college-readiness benchmarks, down from 49% the year prior. For the other section, 68% met benchmarks, a drop of a couple of percentage points.

Hispanic and black students continue to lag behind their white and Asian counterparts on the exam.

About 56% of students taking the test had parents with a college degree. About 9% of students’ parents had no high-school diploma, while 27% had a high-school diploma, but not a college diploma. About 8% had no response regarding parents’ education.

Some colleges and universities have dropped using the SAT and rival ACT as an admission requirement in an attempt to level the playing field for underrepresented students. SAT and ACT administrators have defended use of the exams, saying many colleges and universities rely on the scores to help in the admission process.

About 1,050 accredited, four-year colleges and universities now make decisions about all or most applicants without regard to ACT or SAT test scores, according to the National Center for Fair and Open Testing, known as FairTest.

Robert Schaeffer, public education director of FairTest, noted that the 2019 SAT average scores declined in nearly every demographic group.

“SAT score gaps between demographic groups—when broken down by test-takers’ race, parental education or household income—grew even larger in the high school class of 2019,” Mr. Schaeffer said. “The exam remains a more accurate measure of a test-taker’s family background than of an applicant’s capacity to do college level work. No wonder nearly 40% of all four-year colleges in the country are now test-optional.”

Write to Tawnell D. Hobbs at Tawnell.Hobbs@wsj.com

Corrections & Amplifications
The parents of 56% of students taking the SAT had college degrees. An earlier version of this article incorrectly said that 56% of students whose parents had college degrees took the test. (Sept. 26, 2019)