Next September, students dissatisfied with their ACT scores will be able for the first time to retake a selected portion of the college admission test without having to repeat the entire exam.

The policy change, announced Tuesday, is a major development for a high-stakes testing ritual that has stirred angst among generations of college-bound students. It means that a student who aced three of the ACT’s four sections could choose to focus solely on raising the score of that troublesome fourth section without fear of getting lower results on the other three. Never in the 60-year history of the ACT has there been an option for a partial retake.

The risk of a lower score on the second or third try has long posed a trade-off for students, parents and counselors who are pondering strategy on retests.

But ACT officials said they want to ease those concerns because they have concluded that the test’s integrity remains intact whether it is administered in whole or in part.

“Our research shows that ACT scores for students who take individual section tests are consistent with those earned when they take the entire test,” Suzana Delanghe, ACT chief commercial officer, said in a statement. “We are simply offering new ways to take the ACT, saving students time and giving them the ability to focus only on subject areas needing improvement.”

The ACT, with a top score of 36, is one of two major college admission tests. It lasts nearly three hours, with multiple-choice questions covering English, math, reading and science. An optional essay-writing prompt takes 40 minutes more. About 1.9 million U.S. students in the Class of 2018 took the ACT.

The ACT’s rival is the College Board’s SAT (maximum score 1600), which last year reclaimed the lead in market share for admission testing. Selective colleges accept either test. Some students send results from both.

Partial retakes are not allowed with the SAT.

In another significant shift Tuesday, the ACT testing organization disclosed it is starting to offer online testing for students who sit for the exam on Saturdays — enabling them to get results in as few as two business days. Those who take the paper-and-pencil version often wait about three weeks.

The faster that students receive scores, the faster they can use that information in assembling a list of potential colleges and making decisions about test retakes.

Online testing is now the norm for students who take the ACT overseas. Some public school systems — notably in Oklahoma, Arkansas and South Carolina — administer the ACT online during school days. ACT chief executive Marten Roorda said online testing offers security advantages. “Paper is something that can get stolen or lost,” he said.

Paper-and-pencil testing will remain widely available, the ACT said, but the digital method will expand over time.

In a third policy shift, ACT will enable students, starting in September, to choose to send colleges an official report, drawn on results from multiple tries at the test, known as a “superscore.”

Used by many colleges, superscoring credits students with their best result on each of the four sections if they have been tested more than once. Many colleges allow applicants to self-report ACT or SAT scores, using the superscoring format. But the ACT’s policy shift will give the format additional legitimacy through official score reports.

Superscoring will not be the only option for official reports. Some universities ask students to send the results of all testing sessions. Others ask for the result of their best testing session.

ACT officials said their research recently found that superscoring does not reduce the power of the test to predict how students will perform when they enter college. Researchers reported in July that superscoring may be a better predictor than other ways of communicating test results.

Those students “who are willing to forgo multiple Saturdays to sit for a multiple-hour test with the hope of maybe increasing their score are also the students who are likely to ask questions in their college courses, visit their professor during office hours, and take advantage of any extra credit opportunities to ensure the best possible grade,” ACT researchers Krista Mattern and Justine Radunzel wrote.

ACT officials shared details of the initiatives with reporters Monday on the condition that they not contact outsiders for comment until the announcement became public early Tuesday.

Roorda said the ACT consulted with colleges, educators, parents and students as it developed the changes. There was especially strong support for allowing students to target specific sections for retesting, he said. “We want to make it a better experience and create more options,” he said. ­Roorda said he expects that taking part of the test will cost less than taking the full version. Exactly how much less remains to be decided.

This school year, the standard ACT charge is $52 per session, or $68 with the writing option. Fee waivers are available to qualified students in financial need.

Under the ACT’s new policy, students must take the full test before they can choose a partial retake of one or more sections. Retakes of specific sections, including the optional writing test, will be available only through online testing centers, not in the paper-and-pencil format. Students with disabilities who qualify for extended time or other testing accommodations on the ACT will also be able to use the partial-retake option.