RALEIGH, N.C. — Helping students with their homework, it turns out, can be big business.

That’s how Advanced Instructional Systems has quietly become one of N.C. State University’s largest and fastest-growing spinoffs.

Now the company is making plans to build on its basic online service, using information it learns about each student from answers they get wrong to create a form of instant, customized tutoring that’s available around the clock.

The company, which offers an online assignment and testing service for university and high school students called WebAssign, was started by a small group of academics on campus in the late 1990s. The university holds rights to part of the underlying computer coding and company trademark and licenses those to the company, which is owned by a handful of its employees.

AIS moved from the main campus onto the public-private Centennial Campus in 2003 with just a dozen employees, said John S. Risley, a physics professor and the company’s chief executive. It has grown to nearly 150 employees now — many of them with science, math and engineering degrees — and in the year that just ended passed two milestones: its 5 millionth student user and one billionth answer processed for homework assignments, tests and practice problems.

In any given semester now, more than half a million students are using WebAssign.

It has grown so much that in December, Risley and the company’s other owners felt it was time to appoint a board of directors, he said.

“When you are a startup and you grow quickly, it becomes clear you need the help and experience,” Risley said.

If the company ever goes public, NCSU would get a share of the money raised, but that may not happen, at least not any time soon.

In the tech world, many private companies seem to be aimed at going public from the minute they’re founded, but Risley said there aren’t any plans for that. He said the owners are interested in education and like the direction the company’s going.

For now, institutions sign up for WebAssign, and course instructors control the assignments. The company plans to begin marketing directly to students and to offer them an enhanced service that verges on using artificial intelligence to craft an online tutor. The company can have access to hundreds or even thousands of answers for a given student, Risley said, and can sift and monitor that data to tailor their learning.

Analyzing several questions students get wrong in a physics class, for example, can show whether they’re getting an answer wrong because they don’t understand the concept or they have a deficiency in a specific math skill. If they happened to have used WebAssign for, say, an algebra class a semester earlier, that would further expand the information that the system has for analyzing what they know and what they need to work on. The company’s current service gets solid reviews from the instructors who use it in their classes.

“They got in fairly early, I think, and as a consequence developed what is probably the most mature system for online homework of this type that’s available,” said David Pengra, a senior lecturer at the University of Washington’s physics department, where he oversees graduate assistants who teach up to 50 sections of introductory undergraduate physics labs.

His department uses WebAssign for lectures and labs for the course. Pengra said the department started using it in the labs about two years ago, just in time to help with a sharp budget cut from the state. It let them cut the 400 hours of graduate assistant time by about one-third just when they badly needed the savings to preserve teaching quality. Most of the saved labor was in that most mind-numbing aspect of teaching: grading assignments.

At Penn State, the company’s service is used in seven large physics classes with a total of 3,500 students. It freed the 60 or so graduate teaching assistants to do substantially more teaching because they no longer have to grade a couple million homework answers each semester, said John Hopkins, a senior instructor.

It also allowed the university to boost the staffing of a night-time learning lab where students can come for help from one or two graduate assistants to eight or nine, all without additional cost. “And that lab gets used a lot,” he said.

Students benefit, too, he said, because they get instant feedback on their answers and, the way instructor there configure WebAssign, can try five more times for a diminishing amount of credit. That instant feedback makes it easier for students to know whether they have grasped a concept or not, he said.

“That’s really important, because they know whether they’ve made a mistake immediately, instead of having to turn in the assignment and wait a few days or more to get it back,” he said. “You can imagine how important that might be if they have an exam on the material the next day.”

Given graduate assistants’ lack of experience, the instructors said, reducing their role in grading yields results that are more reliable.

Part of the system’s strength is that it relies on an approach that gives instructors flexibility in which textbooks they use and how they can formulate assignment questions, Pengra said. Some competitors are tied to a specific publisher and only offer a single text for a given class.

AIS includes 26 job openings. In particular it has been a boon for those with math, engineering, computer science and science degrees.

University officials declined to say what the company pays in licensing fees, but the total from all start-ups is about $5.1 million a year. The fees, while obviously welcome, aren’t the university’s goal for startups, said Kelly B. Sexton, an assistant director in the office of technology transfer. The main point is jobs.

NCSU’s startups have created more than 6,900 jobs, including 3,100 in North Carolina, she said. That’s up from 3,071 total jobs, including about 2,500 in the state, in 2009.