Last summer, as he approached his crucial junior year in high school, Larry Cohen knew he faced a problem: the upcoming Scholastic Aptitude Tests.

Larry was a good student who got mostly B's at Rockville High School, but he had trouble with math and knew he needed help. So he and his parents made a decision that more and more families are making: They hired a private tutor.

Barbara Cohen, Larry's mother, said the family "never thought twice" about hiring a tutor.

"You want your child to achieve a certain level," she said, "and competition for colleges today is so great. If you can help him get into the best school you can, you do it."

While figures for the number of students who receive private tutoring are difficult to obtain, educators say the tutor business is booming.

"You can go down the block in many suburban neighborhoods and every other house has a kid being tutored," said Sue Gurland, director of Traveling Tutors Inc., a Silver Spring-based company that tutored more than 700 students last year, including Larry Cohen. The Maryland Yellow Pages alone lists nearly 30 private tutoring firms, almost twice as many as five years ago, and experts expect the boom to continue.

One reason is the changing nature of who is being tutored. Traditionally, private tutors worked with students who were doing poorly in school. These days, tutors often are working with good students who want to do even better.

"We used to work mainly with kids who were falling behind," said Gurland, a former French teacher who founded the company in 1977. "Now, we get a lot of kids who want to get ahead of the game -- who have a tutor [help them] in geometry before they take it, for instance."

Larry Cohen, now 17, said tutoring "was the best thing for me. I knew I needed help and my teachers just couldn't help me. I just didn't understand the way they were teaching me . . . . With math, sitting down and looking at a problem for an hour won't help if you don't understand it. But my tutor, Mr. [John] Mathews, came to my house and went over whatever problem I had."

Cohen, who said he "really worried about the SATs" before he took them, is happy with his scores. Both he and his parents say they would hire a tutor again if necessary.

Another reason tutoring is becoming popular, parents and students say, is that schools -- both public and private -- often do not give struggling students the individual attention they need.

"My daughter's teachers have a couple hundred kids to deal with," said Kay Meek, whose daughter attends Montgomery Blair High School in Silver Spring. "It's hard for them to see if one is floundering. They just don't have the time."

Tutoring has helped Kimberly, 16, with geometry, said Meek, but also "helped her feel good about all her subjects. It helped her realize her full potential. She even got an A in geometry one marking period."

Adds Susan Monroe, a writer whose daughter Nicoletta, 17, a junior at Holton-Arms School in Bethesda, was tutored in algebra last year, "Like all parents, I want the best education my child can get, and if I have to pay a tutor, I'll do it."

Hiring a private tutor isn't cheap. Area firms charge $17 to $26 an hour. Generally, students are tutored for an hour or two at a time, once or twice a week. But parents say hiring a tutor is worthwhile.

"When you need intensive tutoring as my daughter did, you expect to pay for it," Monroe said. "The point is for your child to do well, have a good self-image and learn the stuff . . . . By God, she passed algebra."

Traveling Tutors employs about 200 tutors, and Tutors Inc. of Rockville, about 100. Tutors for both firms are independent contractors who may also work for other tutoring services. Most are current or former teachers who work part time, though some, like John Mathews, are full-time tutors.

Tutors say they are attracted to the job by good pay and the chance to work one-on-one with students. John Mathews, who tutors as many as 20 students simultaneously, said "I love to teach, and I like the variety. I teach many different age levels and subjects, and I have very little administrative work and no discipline problems."

While educators don't question the value of tutoring a student who otherwise might fail a course, some of them believe that hiring a private tutor to help a B student get A's is unnecessary, especially when schools themselves offer assistance. Private tutors, critics say, prey on the fears of achievement-oriented families.

"Personally, I feel there is often much too much pressure on children to achieve," said Renee Brimfield, coordinator of new program development for Montgomery County schools. "Of course, we want kids in our school system to excel, but we have the resources to help people who need it."

Montgomery County offers many forms of help for those who need it, said Brimfield, including reading specialists, math specialists and guidance counselors for academic and psychological problems.

Critics also say that because of the high cost, students who receive tutoring in preparation for standardized tests such as the SAT have an unfair advantage over those who can't afford it.

Says Gurland, "I'd love to see the information we give about SATs available to everyone without people having to pay for it, but the reality is that they do [have to pay for it]."

The firms also point out that not all clients are well-to-do. Robert Miller, director of Tutors Inc. of Rockville said, "We have students throughout the area, from the inner-city to Potomac." Many families have to scrape together money to pay for the tutoring, he said. "Obviously they think it's worth the investment."

Adds Gurland: "Tutoring is not a luxury like horseback riding or a VCR; it's an investment. If tutoring can help a kid who sees herself as a B student break through to get A's, we've done a lot for that kid's self-concept."