Learn to code. Get a job. Then what?

Dozens of coding "boot camps" have popped up from New York and San Francisco to Omaha and Albuquerque in the past two years. Driven by a robust hiring market for people fluent in programming languages such as Python and Ruby on Rails, these programs have fueled hopes—and demand—among underemployed 20-somethings and others looking for a sure-thing career.

But as the programs, which charge anywhere from $6,000 to nearly $18,000 for a three-month course, have proliferated, so has concern about their ability to graduate career-ready coders in less than one college semester.

To maintain their reputations and manage expectations among students and employers, some schools are extending their curricula and cutting class sizes. They're also adding a dose of caution to their sales pitches, advising that even superstar students will need to go beyond the entry-level instruction to keep up in a fast-changing industry.

"I don't think you can be a career-ready programmer in nine or 10 weeks" without prior experience with other aspects of engineering or computer science, said Jeff Casimir, founder of Denver-based Turing School, a seven-month program that costs $17,500.

Jeff Atwood, who has been a software engineer for 20 years and now runs Discourse, a provider of online discussion-forum software, says he worries about a "gold rush mentality" among students and instructors. Wrestling with thousands of lines of code on long-term projects with shifting requirements can be a frustrating endeavor, he says, so the industry isn't a good fit for those seeking quick success.

In their marketing materials, many code schools promise just that: "Novice to Hireable in 12 (Intense) Weeks" from Omaha Code School, and "From amateur to professional in 13 weeks" from New York's Fullstack Academy. But in interviews, founders strike a more cautious tone.

"Our students are accepting risk, and they're putting a lot of trust in us," said Sumeet Jain, founder of Omaha Code School, which graduated its first students this month. He asks applicants to calculate what kinds of jobs they'd need to earn a return on the $6,000 class.

The failure of a few programs could tarnish the whole industry, said Jake Schwartz, chief executive and co-founder of three-year-old General Assembly, one of the first code schools. With programs in New York, Hong Kong and other cities, General Assembly charges $11,500 for a full-time, 12-week Web development class in the U.S., and says its alums have landed at McKinsey & Co., Google Inc. and Spotify Ltd.

Founders stress that they are generally preparing students for entry-level, junior-developer jobs. Web-development jobs, which paid a median salary of $62,500 in 2012, are projected to increase about 20% between 2012 and 2022, according to the Labor Department.

"We're not promising you're going to make $120,000 after three months," said Peter Barth, CEO of the Iron Yard, a school in Greenville, S.C., that is expanding into more than a dozen cities. The Iron Yard guarantees graduates a job within six months of finishing its three-month course, and it reports a 100% success rate so far, thanks in part to having a director at each campus who matches students with employers.

Schools that release outcomes data show impressive figures—many schools tout job-placement rates topping 90% within three months—and graduates say they're pleased with the training. But absent industry standards, such as whether internships and temp jobs should count in placement rates, schools' numbers can be difficult to compare, say some students and school operators.

And among those who do land jobs, the education doesn't end at coding school. Tech employers say that code-school grads often need mentoring on the job—such as being paired with a more senior developer—as well as more formal course work and self-teaching. That's true for more experienced professionals as well because the requisite skills evolve so rapidly.

Nathan Hanna, an engineering manager at Charleston, S.C.-based software firm Benefitfocus Inc., says he's impressed with the skills Iron Yard grads pick up in three months. Still, he notes that "you need a good onboarding and mentoring process to bring them along." He hired one Iron Yard coder as a user-interface engineer and is now coaching her to troubleshoot more complex software issues.

After completing a three-month, $8,000 Starter League coding course in Chicago last spring, Jem Hilton earned $1,500 a month during a four-month apprenticeship at Chicago-based health-care startup Purple Binder Inc. But that stint helped the former philosophy Ph.D. student land a full-time developer position at Northwestern University that pays enough for him to quit his side job tending bar.

Not everyone starts on the bottom rung. Mark Wilbur was hired at deals website Groupon Inc. after completing a 12-week, $17,780 stint at Hack Reactor in San Francisco last year. He says he contributed usable code within three days of arriving, and his signing bonus was more than his 2012 income as a freelance front-end developer. Mr. Wilbur quit the job recently to pursue a startup with two Hack Reactor classmates.

Employers and even code-school partisans aren't certain that short courses for novices can solve their talent problems.

Dev Bootcamp, with campuses in Chicago, San Francisco and New York, now requires participants to complete nine weeks of preparatory material on coding basics before classes start. And MakerSquare, in Austin, has shrunk its class size, added two weeks of course work and limited enrollment to students with some prior programming experience.

Mark Rickmeier, chief operating officer of Web and mobile design firm Table XI Partners LLC in Chicago, hired an entry-level developer last year from a six-month-long program. The woman's commitment to months, not weeks, of instruction proved she wasn't just pursuing a career in coding on a whim, he said.

The employee is still with the company, he added, and he considers her a successful hire.

Indiegogo Inc., a crowd-funding company in San Francisco, has been "cherry-picking" novice coders from schools like Dev Bootcamp and Hackbright, but it has mostly hired individuals with engineering degrees, said Will Haines, director of engineering. The startup is considering creating an apprenticeship program for code-school grads who are "promising but technically raw," he said.

Such apprenticeships, which are gaining traction as a transitional model for many code schools and their partner employers, attempt to balance companies' need for developers with recognition that graduates need more formal learning.

"There's clearly a concern in the industry: 'What do we do with all of these people that clearly need more help and don't have the opportunity to get it?'" Mr. Rickmeier said.

Write to Melissa Korn at melissa.korn@wsj.com and Lauren Weber at lauren.weber@wsj.com

Corrections & Amplifications

Mark Wilbur paid $17,780 to attend a 12-week coding bootcamp at Hack Reactor. A Marketplace article on Wednesday about coding schools incorrectly said he paid $11,500. (May 20, 2014)